balance_region: remove the unnecessary retry when try to pick a region to transfer

[AndreMouche]

What problem does this PR solve?

Issue Number: Close #5397

What is changed and how does it work?

Check List

Tests

benchmark

current-branch

wuxuelian@fun-2 ~/go/pd/server/schedulers                                                                                                                                                                    [14:18:23] 
> $ go test -benchmem -run=^$ -bench ^Benchmark  -benchtime=100x                                                                                                                            [±balance_region_speedup ●]
goos: darwin
goarch: amd64
pkg: github.com/tikv/pd/server/schedulers
cpu: Intel(R) Core(TM) i7-8850H CPU @ 2.60GHz
BenchmarkPlacementRule-12  100          50000943 ns/op        24047141 B/op     708799 allocs/op
BenchmarkLabel-12                    100           7949532 ns/op         2518519 B/op      28179 allocs/op
BenchmarkNoLabel-12                  100           8786278 ns/op         1570206 B/op      21677 allocs/op
BenchmarkTombStore-12                100           8777006 ns/op         1570173 B/op      21677 allocs/op

master

> $ go test -benchmem -run=^$ -bench ^Benchmark  -benchtime=100x                                                                                                                                            [±master ●]
goos: darwin
goarch: amd64
pkg: github.com/tikv/pd/server/schedulers
cpu: Intel(R) Core(TM) i7-8850H CPU @ 2.60GHz
BenchmarkPlacementRule-12    100          52804382 ns/op        24047187 B/op     708797 allocs/op
BenchmarkLabel-12                    100           9142366 ns/op         2518413 B/op      28176 allocs/op
BenchmarkNoLabel-12                  100           9116922 ns/op         1570108 B/op      21674 allocs/op
BenchmarkTombStore-12                100           9027190 ns/op         1570099 B/op      21674 allocs/op

• Unit test

Release note

 None.

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[codecov[bot]]

Codecov Report

:x: Patch coverage is 94.44444% with 2 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 75.75%. Comparing base (99e38eb) to head (0d6711b). :warning: Report is 1913 commits behind head on master.

Files with missing lines	Patch %	Lines
server/schedule/filter/region_filters.go	81.81%	1 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5398      +/-   ##
==========================================
+ Coverage   75.58%   75.75%   +0.16%     
==========================================
  Files         312      312              
  Lines       31061    31074      +13     
==========================================
+ Hits        23479    23541      +62     
+ Misses       5572     5534      -38     
+ Partials     2010     1999      -11     

Flag	Coverage Δ
unittests	75.75% <94.44%> (+0.16%)	:arrow_up:

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[ti-chi-bot]

@AndreMouche: PR needs rebase.

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