planner: remove the useless variable tidb_enable_point_get_cache

[qw4990]

What problem does this PR solve?

Issue Number: close #36598

Problem Summary: planner: remove the useless variable tidb_enable_point_get_cache

What is changed and how it works?

planner: remove the useless variable tidb_enable_point_get_cache

Check List

Tests

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

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

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Reminiscent
• Yisaer

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[ti-chi-bot]

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[Reminiscent]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: a4d2c12

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/2a11a1e644db8ddc99847e82be4a1c9831dbbb66

[qw4990]

/merge

[ti-chi-bot]

@dayicklp: Thanks for your review. The bot only counts approvals from reviewers and higher roles in list, but you're still welcome to leave your comments.

In response to this:

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.

[qw4990]

/merge

[qw4990]

/merge

[sre-bot]

TiDB MergeCI notify

🔴 Bad News! New failing [1] after this pr merged.
These new failed integration tests seem to be caused by the current PR, please try to fix these new failed integration tests, thanks!

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci-tidb/integration-common-test	🟥 failed 1, success 16, total 17	18 min	New failing
idc-jenkins-ci-tidb/tics-test	🔴 failed 1, success 0, total 1	7 min 42 sec	Existing failure
idc-jenkins-ci-tidb/integration-ddl-test	✅ all 6 tests passed	8 min 53 sec	Fixed
idc-jenkins-ci/integration-cdc-test	🟢 all 36 tests passed	33 min	Existing passed
idc-jenkins-ci-tidb/common-test	🟢 all 11 tests passed	14 min	Existing passed
idc-jenkins-ci-tidb/mybatis-test	🟢 all 1 tests passed	11 min	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-1	🟢 all 26 tests passed	10 min	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	8 min 35 sec	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	6 min 25 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed

[morgo]

There is a compatibility feature for removed variables here: https://github.com/pingcap/tidb/blob/master/sessionctx/variable/removed.go -- did you want to add this?