Remove magic monitor support for ModelCheckpoint

[carmocca]

What does this PR do?

Remove deprecated support for relying on the magic monitor val_loss. save_top_k does not need to be Optional anymore.

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[codecov]

Codecov Report

Merging #8293 (6afb295) into master (fc12fe7) will increase coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #8293    +/-   ##
=======================================
+ Coverage      88%     92%    +4%     
=======================================
  Files         213     213            
  Lines       13808   13797    -11     
=======================================
+ Hits        12151   12730   +579     
+ Misses       1657    1067   -590     

[tchaton]

LGTM !