tools: fix panic when using stores show limit

[rleungx]

What problem does this PR solve?

If we firstly use stores remove-tombstone and then calling stores show limit in pd-ctl. It will panic due to the nil pointer.

What is changed and how it works?

This PR fixes it by removing the store limit when removing a tombstone store.

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch
• Need to be included in the release notes

[codecov-io]

Codecov Report

Merging #1807 into master will increase coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1807      +/-   ##
==========================================
+ Coverage   77.95%   78.13%   +0.17%     
==========================================
  Files         167      167              
  Lines       16468    16473       +5     
==========================================
+ Hits        12838    12871      +33     
+ Misses       2592     2562      -30     
- Partials     1038     1040       +2

Impacted Files	Coverage Δ
server/schedule/operator_controller.go	90.99% <100%> (+0.08%)	⬆️
server/cluster.go	84.62% <100%> (-0.24%)	⬇️
pkg/etcdutil/etcdutil.go	84.05% <0%> (-4.35%)	⬇️
server/member/leader.go	77.55% <0%> (-3.58%)	⬇️
server/schedule/operator/operator.go	84.98% <0%> (-0.55%)	⬇️
server/grpc_service.go	57.91% <0%> (+0.21%)	⬆️
server/server.go	82.6% <0%> (+0.56%)	⬆️
server/handler.go	52.14% <0%> (+0.75%)	⬆️
pkg/btree/btree.go	86.14% <0%> (+0.8%)	⬆️
... and 6 more

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[lhy1024]

LGTM. do we need to add unit test about it?

[lhy1024]

LGTM

[sre-bot]

/run-all-tests

[sre-bot]

@rleungx merge failed.

[rleungx]

/merge

[sre-bot]

/run-all-tests

[sre-bot]

cherry pick to release-3.0 in PR #1808

[sre-bot]

cherry pick to release-3.1 in PR #1809