Dashboard: fix a request mapping issue of removing machines api

[jasonjoo2010]

Describe what this PR does / why we need it

The api {app}/machine/remove.json is called through GET in old version but it changed to POST now and definition of the mapping was not changed together.

[codecov-io]

Codecov Report

Merging #559 into master will decrease coverage by 0.36%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #559      +/-   ##
============================================
- Coverage     38.68%   38.31%   -0.37%     
+ Complexity     1166     1159       -7     
============================================
  Files           267      267              
  Lines          8414     8414              
  Branches       1133     1133              
============================================
- Hits           3255     3224      -31     
- Misses         4742     4777      +35     
+ Partials        417      413       -4

Impacted Files	Coverage Δ	Complexity Δ
...a/csp/sentinel/slots/statistic/base/Striped64.java	26.04% <0%> (-23.96%)	5% <0%> (-2%)
...a/csp/sentinel/slots/statistic/base/LongAdder.java	17.02% <0%> (-14.9%)	4% <0%> (-4%)
...ava/com/alibaba/csp/sentinel/node/ClusterNode.java	95.45% <0%> (-4.55%)	7% <0%> (-1%)

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[sczyh30]

LGTM

[sczyh30]

Thanks