*: remove EnablePointGetCache and cacheDB

[Defined2014]

What problem does this PR solve?

Issue Number: close #48958

Problem Summary:

We can only enable EnablePointGetCache by golang code rather than a global variable (like test used). And its default value is false which means it nerver be used anytime.

Also we could remove cacheDB and cacheBatchGetter etc.

The realed global variable is deleted by #36844.

What changed and how does it work?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

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[codecov]

Codecov Report

Merging #48962 (686425b) into master (1456c9d) will increase coverage by 1.6460%.
Report is 3 commits behind head on master.
The diff coverage is 100.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #48962        +/-   ##
================================================
+ Coverage   70.9941%   72.6401%   +1.6460%     
================================================
  Files          1368       1389        +21     
  Lines        404987     411092      +6105     
================================================
+ Hits         287517     298618     +11101     
+ Misses        97448      93616      -3832     
+ Partials      20022      18858      -1164     

Flag	Coverage Δ
integration	43.6201% <100.0000%> (?)
unit	70.9932% <ø> (-0.0009%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	53.9663% <ø> (ø)
parser	∅ <ø> (∅)
br	48.8110% <ø> (-4.2698%)	⬇️