config: completely remove the deprecated field from the PD server config (#8981)

[ti-chi-bot]

This is an automated cherry-pick of #8981

What problem does this PR solve?

Issue Number: Close #8980

What is changed and how does it work?

server/config: completely remove the deprecated field from the PD server config

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

From 7.1.1 to master:

master restart twice:

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-01-09 09:50:07.109650186 +0000 UTC m=+433550.398481891: ☑️ agreed by rleungx.
• 2025-01-09 09:51:35.959076936 +0000 UTC m=+433639.247908634: ☑️ agreed by nolouch.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 75.54%. Comparing base (43949da) to head (089d715).
Report is 1 commits behind head on release-8.5.

Additional details and impacted files

@@               Coverage Diff               @@
##           release-8.5    #8985      +/-   ##
===============================================
- Coverage        75.61%   75.54%   -0.07%     
===============================================
  Files              462      462              
  Lines            72365    72354      -11     
===============================================
- Hits             54720    54663      -57     
- Misses           14151    14185      +34     
- Partials          3494     3506      +12     

Flag	Coverage Δ
unittests	75.54% <50.00%> (-0.07%)	⬇️

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: niubell, nolouch, rleungx

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Needs approval from an approver in each of these files:

• OWNERS [nolouch,rleungx]
• server/config/OWNERS [niubell]

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