Fix VARYING syntax

[zier-one]

Index Issue: pingcap/tidb#11486

These are bad (incompatible) SQL cases:

ALTER TABLE t_n STORAGE DISK , MODIFY ident NATIONAL VARCHARACTER ( 12 ) COLUMN_FORMAT FIXED FIRST
ALTER TABLE d_n.t_n MODIFY COLUMN ident LONG CHAR VARYING AFTER ident REMOVE PARTITIONING
ALTER TABLE t_n CHANGE COLUMN ident ident LONG CHAR VARYING BINARY CHARSET text_string FIRST , TABLESPACE ident
ALTER TABLE d_n.t_n ALGORITHM = DEFAULT , DROP ident CASCADE PARTITION BY RANGE FIELDS ( ident ) PARTITIONS 10 ( PARTITION ident VALUES LESS THAN MAXVALUE ENGINE ident )

Claim this subtask based on the comments below this issue.

[lauhg]

Let me fix it.

[lauhg]

ALTER TABLE d_n.t_n ALGORITHM = DEFAULT , DROP ident CASCADE PARTITION BY RANGE FIELDS ( ident ) PARTITIONS 10 ( PARTITION ident VALUES LESS THAN MAXVALUE ENGINE ident )

This case does not belong to VARYING syntax cases. And its syntax is good, but errors occur when running tests.

parser_test.go:2516:
    s.RunTest(c, table)
parser_test.go:318:
    c.Assert(err, IsNil, comment)
... value *errors.withStack = [ddl:1484]Wrong number of partitions defined, mismatch with previous setting ("[ddl:1484]Wrong number of partitions defined, mismatch with previous setting")
... source alter table d_n.t_n algorithm = default, drop ident cascade partition by range fields (ident) partitions 10 (partition ident values less than maxvalue engine ident)

[zier-one]

@lauhg remove it. it's my carelessness