remove deprecated code

[wnbts]

Description of changes: remove unused deprecated code in model manager.

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[codecov]

Codecov Report

Merging #228 into master will decrease coverage by 0.19%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #228      +/-   ##
============================================
- Coverage     72.40%   72.20%   -0.20%     
+ Complexity     1289     1276      -13     
============================================
  Files           139      139              
  Lines          6073     6045      -28     
  Branches        469      469              
============================================
- Hits           4397     4365      -32     
- Misses         1464     1466       +2     
- Partials        212      214       +2     

Flag	Coverage Δ	Complexity Δ
#cli	80.30% <ø> (ø)	0.00 <ø> (ø)
#plugin	71.12% <ø> (-0.23%)	1276.00 <ø> (-13.00)

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Impacted Files	Coverage Δ	Complexity Δ
...opendistroforelasticsearch/ad/ml/ModelManager.java	90.93% <ø> (-1.13%)	100.00 <0.00> (-12.00)
...asticsearch/ad/cluster/ADClusterEventListener.java	88.00% <0.00%> (-4.00%)	13.00% <0.00%> (-1.00%)