Fix: gettyRPCClientPool.remove deadlock

[pantianying]

What this PR does:

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-io]

Codecov Report

Merging #183 into master will decrease coverage by 0.2%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #183      +/-   ##
==========================================
- Coverage   66.48%   66.28%   -0.21%     
==========================================
  Files          90       90              
  Lines        5541     5540       -1     
==========================================
- Hits         3684     3672      -12     
- Misses       1465     1476      +11     
  Partials      392      392

Impacted Files	Coverage Δ
protocol/dubbo/pool.go	59.01% <100%> (-0.23%)	⬇️
cluster/cluster_impl/base_cluster_invoker.go	54.71% <0%> (-16.99%)	⬇️
registry/etcdv3/registry.go	55.74% <0%> (-1.15%)	⬇️

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[AlexStocks]

pls change the target branch from master to develop.

[AlexStocks]

LGTM. @hxmhlt pls review and merge it.

[fangyincheng]

LGTM