Fix for get detector API

[kaituo]

Issue #, if available:

Description of changes:

We didn't return after sending back channel responses after one recent change. Later, when we use the channel to send back responses again, we get " java.lang.IllegalStateException: Channel is already closed". This PR fixes this issue.

This PR also removes a redundant setting.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

Merging #290 into master will decrease coverage by 0.10%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #290      +/-   ##
============================================
- Coverage     71.39%   71.29%   -0.11%     
  Complexity     1871     1871              
============================================
  Files           195      195              
  Lines          9020     9018       -2     
  Branches        764      764              
============================================
- Hits           6440     6429      -11     
- Misses         2215     2224       +9     
  Partials        365      365              

Flag	Coverage Δ	Complexity Δ
#cli	79.27% <ø> (ø)	0.00 <ø> (ø)
#plugin	70.59% <100.00%> (-0.12%)	1871.00 <0.00> (ø)

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Impacted Files	Coverage Δ	Complexity Δ
...stroforelasticsearch/ad/AnomalyDetectorPlugin.java	93.44% <ø> (ø)	13.00 <0.00> (ø)
...ticsearch/ad/settings/AnomalyDetectorSettings.java	100.00% <ø> (ø)	1.00 <0.00> (ø)
...d/transport/GetAnomalyDetectorTransportAction.java	44.32% <100.00%> (-2.55%)	9.00 <0.00> (ø)
...port/SearchAnomalyDetectorInfoTransportAction.java	54.54% <0.00%> (-9.10%)	4.00% <0.00%> (ø%)
...ransport/DeleteAnomalyDetectorTransportAction.java	57.95% <0.00%> (-2.28%)	16.00% <0.00%> (ø%)