pkg/lightning : remove get_regions call in physical backend (#46202)

[ti-chi-bot]

This is an automated cherry-pick of #46202

What problem does this PR solve?

Problem Summary:
When lightning runs in physical import mode, it runs this sql query

SELECT REGION_ID, APPROXIMATE_SIZE FROM information_schema.TIKV_REGION_STATUS WHERE TABLE_ID = ?

This call is made by each lightning process in parallel import mode and by multiple worker threads in a lightning process. It can potentially overload the PD if number of regions are too many. However, it appears that this call may be unnecessary since its purpose is to display a warning message and increment a counter in the SplitAndScatterRegionByRanges() function. Considering that lightning physical import always operates on a new table, splitting the new region should not pose any harm.

Issue Number: close #45507

What is changed and how it works?

Remove the getRegions call from lightning

Check List

Tests

• Manual test (add detailed scripts or steps below)

1. Run lightning in parallel import mode
2. Check get_Regions call in PD dashboard. It should be 0
3. Lightning should run successfully

Release note

We saw this issue in 6.5.0

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

None

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Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/unit-test	d163cb0	link	true	/test unit-test
idc-jenkins-ci-tidb/check_dev	d163cb0	link	true	/test check-dev

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