remove json_unquote() from expressions-pushed-down.md (#5564)

[ti-srebot]

cherry-pick #5564 to master
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---

I removed JSON_UNQUOTE() because this expression is not currently pushed down.

pingcap/tidb@721782d

MySQL [test]> explain analyze select count(*) from t2 where json_unquote(json_extract(j, '$[0]')) < 10;
+-----------------------------+---------+---------+-----------+---------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------------------------------------------------------------------+-----------+------+
| id                          | estRows | actRows | task      | access object | execution info                                                                                                                                                                                              | operator info                                                           | memory    | disk |
+-----------------------------+---------+---------+-----------+---------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------------------------------------------------------------------+-----------+------+
| StreamAgg_7                 | 1.00    | 1       | root      |               | time:1.36ms, loops:2                                                                                                                                                                                        | funcs:count(1)->Column#3                                                | 636 Bytes | N/A  |
| └─Selection_11              | 1.60    | 2       | root      |               | time:1.35ms, loops:2                                                                                                                                                                                        | lt(cast(json_unquote(cast(json_extract(cast(test.t2.j), "$[0]")))), 10) | 628 Bytes | N/A  |
|   └─TableReader_13          | 2.00    | 2       | root      |               | time:1.3ms, loops:3, cop_task: {num: 1, max: 1.24ms, proc_keys: 2, rpc_num: 1, rpc_time: 1.22ms, copr_cache_hit_ratio: 0.00}                                                                                | data:TableRangeScan_12                                                  | 219 Bytes | N/A  |
|     └─TableRangeScan_12     | 2.00    | 2       | cop[tikv] | table:t2      | tikv_task:{time:0s, loops:1}, scan_detail: {total_process_keys: 2, total_keys: 4, rocksdb: {delete_skipped_count: 0, key_skipped_count: 2, block: {cache_hit_count: 0, read_count: 0, read_byte: 0 Bytes}}} | range:[0,+inf], keep order:false, stats:pseudo                          | N/A       | N/A  |
+-----------------------------+---------+---------+-----------+---------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------------------------------------------------------------------+-----------+------+
4 rows in set, 1 warning (0.002 sec)

MySQL [test]> show warnings;
+---------+------+----------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                      |
+---------+------+----------------------------------------------------------------------------------------------+
| Warning | 1105 | Scalar function 'json_unquote'(signature: JsonUnquoteSig) can not be pushed to storage layer |
+---------+------+----------------------------------------------------------------------------------------------+
1 row in set (0.000 sec)

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[ti-srebot]

@kolbe please accept the invitation then you can push to the cherry-pick pull requests.
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[morgo]

/LGTM

[TomShawn]

/lgtm

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[TomShawn]

/merge

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This pull request has been accepted and is ready to merge.

Commit hash: 399c5ec