Remove shown_warnings dead code from Trainer

[kaushikb11]

What does this PR do?

Remove shown_warnings dead code from Trainer

Does your PR introduce any breaking changes? If yes, please list them.

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[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #9230 (b2e31c7) into master (f6614b3) will increase coverage by 0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #9230    +/-   ##
=======================================
  Coverage      92%     92%            
=======================================
  Files         176     176            
  Lines       14805   15070   +265     
=======================================
+ Hits        13658   13923   +265     
  Misses       1147    1147