Delete unused global variables from mysqld.h

[robinnewhouse]

Description

Several variables declared in mysqld.h appear to be old system variables
that have been left over after deprecation. Delete them using IDE
refactoring to automatically search for other uses. Most cases had no
other uses in the code.

slave_allow_batching had a test that was effectively unused, as the
result was only
-ERROR HY000: Unknown system variable 'slave_allow_batching'
so that was deleted as well.

Build and test still works without issue as expected.

Examples of variables that were deleted and not cleaned up:

• 0c571f8 removed the last usage of VERS_ALTER_HISTORY_KEEP but did not delete it from the enum.
• 7c58e97 moved key_memory_quick_index_merge_root (and similar) to mysqld.cc to mysqld.h, but still was not used anywhere.
• 23d8586 moved key_thread_kill_server along with others to header file, but still was used nowhere. I can't actually find where it was ever used except being defined in a header.

Release Notes

None

How can this PR be tested?

Building the server and running the MTR test suite shows no failures.

Basing the PR against the correct MariaDB version

• This is a bug fix and the PR is based against the earliest maintained branch in which the bug can be reproduced.

Note: As discussed with @LinuxJedi, this is targeting the earliest maintained branch to ease merging upwards, even though it is not necessarily a bugfix.

PR quality check

• I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.

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