schedule: fix panic caused by removing tombstone (#2015)

[rleungx]

What problem does this PR solve?

cherry-pick #2015 to release-3.1 branch

What is changed and how it works?

This PR fixes the panic problem by resetting the attached function when removing the store limit.

Check List

Tests

• Unit test

Related changes

• Need to be included in the release notes

[codecov-io]

Codecov Report

Merging #2038 into release-3.0 will decrease coverage by <.01%.
The diff coverage is 100%.

@@               Coverage Diff               @@
##           release-3.0    #2038      +/-   ##
===============================================
- Coverage        69.44%   69.43%   -0.01%     
===============================================
  Files              170      170              
  Lines            16768    16769       +1     
===============================================
  Hits             11644    11644              
- Misses            4144     4145       +1     
  Partials           980      980

Impacted Files	Coverage Δ
server/schedule/operator_controller.go	87.19% <100%> (+0.47%)	⬆️
server/util.go	68.15% <0%> (-4.46%)	⬇️
server/region_syncer/client.go	78.94% <0%> (-3.95%)	⬇️
server/leader.go	77.95% <0%> (-2.45%)	⬇️
server/handler.go	52.11% <0%> (+0.49%)	⬆️
server/schedule/operator.go	87.58% <0%> (+0.64%)	⬆️
server/namespace_cluster.go	85.71% <0%> (+3.89%)	⬆️
server/schedulers/shuffle_hot_region.go	65.38% <0%> (+6.41%)	⬆️

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[rleungx]

/run-all-tests