system-variables: remove the default value descriptions for last_xxx variables

[qiancai]

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What is changed, added or deleted? (Required)

This PR removes the default value descriptions for the following last_xxx variables:
last_plan_from_binding
last_plan_from_cache
last_sql_use_alloc

Reason: their values are determined by whether a function is used or not, so the default values are meaningless and confusing.

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 (in Chinese).

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 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: system-variables: remove the default value descriptions for last_xxx variables  docs-cn#19982
• 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

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[LGTM Timeline notifier]

Timeline:

• 2025-03-05 03:13:16.215065747 +0000 UTC m=+411309.343985491: ☑️ agreed by tiancaiamao.