functions-and-operators: remove MySQL Reference Manual links (#21083)

[ti-chi-bot]

This is an automated cherry-pick of #21083

First-time contributors' checklist

• I've signed the Contributor License Agreement, which is required for the repository owners to accept my contribution.

What is changed, added or deleted? (Required)

• Remove reference manual links for functions for which we have our own docs.
• Keep links for functions/operators that are not explained in the TiDB docs yet
• We can keep a link at the top linking to the corresponding page in the MySQL Reference Manual like this: TiDB supports most of the user-level [locking functions](https://dev.mysql.com/doc/refman/8.0/en/locking-functions.html) available in MySQL 8.0.

This was done with sed -E -i -e 's/(^#* )\[(.*)\].*$/\1\2/' path/to/file.md

Which TiDB version(s) do your changes apply to? (Required)

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions.

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)

What is the related PR or file link(s)?

• This PR is translated from: functions-and-operators: remove MySQL Reference Manual links docs#21821
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[qiancai]

/approve

[ti-chi-bot]

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