session: improve collation_server handling

[morgo]

What problem does this PR solve?

Problem Summary:

This revisits #23932 with a simpler fix, which I noticed while performing some other refactoring.

The original problem is that when setting the character_set_server, the collation_server is also set; which leads to the collation no longer being set because it is already set.

Because collation_server also sets the character_set_server, this PR takes the approach of removing the code that sets character_set_server. Both variables will be set with collation_server.

What is changed and how it works?

What's Changed:

Small code improvement, function neutral.

Related changes

• Original PR session: fix global variable collation_server does not take effect in new session #23932

Check List

Tests

• Unit test (covered by existing tests)

Side effects

• None

Release note

• No release note

[xiongjiwei]

lgtm

[wjhuang2016]

After we remove the code that sets character_set_server, if glocal character_set_server is set, session character_set_server will not take effect.

[tiancaiamao]

/LGTM

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• tiancaiamao
• xiongjiwei

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[xhebox]

/merge

[ti-chi-bot]

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

Commit hash: c2f3bea

[ti-chi-bot]

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[morgo]

/run-unit-test