Remove unused post_optimizer_step

[carmocca]

What does this PR do?

No public usage: https://grep.app/search?q=post_optimizer_step&filter[lang][0]=Python
Part of #9287. This hook will have no real use after the optimizer step is moved to the precision plugin.

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

Removes the post_optimizer_step hook.

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[codecov]

Codecov Report

Merging #9746 (f804677) into master (019e69f) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #9746   +/-   ##
======================================
- Coverage      93%     93%   -0%     
======================================
  Files         177     177           
  Lines       15389   15384    -5     
======================================
- Hits        14263   14258    -5     
  Misses       1126    1126           

[tchaton]

LGTM ! quick note: It could be used for federate learning to broadcast cast the updated weights.