Remove deprecated TrainerCallbackHookMixin

[otaj]

What does this PR do?

Removes deprecated TrainerCallbackHookMixin. Also, is a followup to #13807

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

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cc @Borda @tchaton @rohitgr7

[codecov]

Codecov Report

Merging #14401 (ee305c0) into master (03f2f32) will increase coverage by 15%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #14401     +/-   ##
=========================================
+ Coverage      61%      76%    +15%     
=========================================
  Files         332      331      -1     
  Lines       26866    26691    -175     
=========================================
+ Hits        16417    20237   +3820     
+ Misses      10449     6454   -3995