Remove variant type promotion and fix notes in the table.

Author: rdblue

format/spec.md

@@ -238,7 +238,6 @@ Valid primitive type promotions are:
 | `date`           |                              | `timestamp`, `timestamp_ns`                | Promotion to `timestamptz` or `timestamptz_ns` is **not** allowed |
 | `float`          | `double`                     | `double`                                   | |
 | `decimal(P, S)`  | `decimal(P', S)` if `P' > P` | `decimal(P', S)` if `P' > P`               | Widen precision only |
-| _all types_      |                              | `variant` if Variant spec allows the type  | Conversion must produce a Variant containing a single value of the original type |
 
 Iceberg's Avro manifest format does not store the type of lower and upper bounds, and type promotion does not rewrite existing bounds. For example, when a `float` is promoted to `double`, existing data file bounds are encoded as 4 little-endian bytes rather than 8 little-endian bytes for `double`. To correctly decode the value, the original type at the time the file was written must be inferred according to the following table:
 
@@ -256,11 +255,9 @@ Iceberg's Avro manifest format does not store the type of lower and upper bounds
 | `string`         | 8 bytes          | `long` or `string`          | Discard and ignore bounds [1] |
 | `string`         | not 4 or 8 bytes | `string`                    | Use `string` if either bound is not 4 or 8 bytes |
 | `decimal(P, S)`  | _any_            | `decimal(P', S)`; `P' <= P` | |
-| `variant`        | _any_            | Any type                    | Discard and ignore bounds |
 
 Notes:
-1. Allowed promotion from `long` to `timestamptz` makes 8-byte lower and upper bounds ambiguous becuase `long` values are in milliseconds and `timestamptz` values are in milliseconds.
-2. Bounds written for an `int` or `long` column promoted to `string` are ambiguous if both bounds have the same 4 or 8 byte length.
+1. Bounds written for an `int` or `long` column promoted to `string` are ambiguous if both bounds have the same 4 or 8 byte length.
 
 Type promotion is not allowed for a field that is referenced by `source-id` or `source-ids` of a partition field if the partition transform would produce a different value after promoting the type. For example, `bucket[N]` produces different hash values for `34` and `"34"` (2017239379 != -427558391) but the same value for `34` and `34L`; when an `int` field is the source for a bucket partition field, it may be promoted to `long` but not to `string`.