x-server: remove the X Server

[jackysp]

What problem does this PR solve?

X Server is never used in TiDB. It is better to implement it as a plugin as MySQL, due to the plugin framework is supported in TiDB.

What is changed and how it works?

Remove the X Server

Check List

Tests

• Unit test

[jackysp]

/run-all-tests

[tiancaiamao]

LGTM

[lysu]

/run-integraion-common-test tidb-test=pr/718

[lysu]

LGTM

[lysu]

/run-integration-common-test tidb-test=pr/718

[lysu]

/run-all-tests

[jackysp]

/run-unit-test

[codecov-io]

Codecov Report

Merging #9038 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9038      +/-   ##
==========================================
+ Coverage   67.25%   67.26%   +<.01%     
==========================================
  Files         371      371              
  Lines       76195    76195              
==========================================
+ Hits        51245    51250       +5     
+ Misses      20415    20412       -3     
+ Partials     4535     4533       -2

Impacted Files	Coverage Δ
config/config.go	14.54% <ø> (ø)	⬆️
ddl/session_pool.go	82.75% <0%> (-10.35%)	⬇️
util/filesort/filesort.go	76.48% <0%> (+0.94%)	⬆️
ddl/delete_range.go	77.14% <0%> (+2.85%)	⬆️

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