Add deprecation warning to ModelCheckpoint when logging val_loss with no monitor

[carmocca]

What does this PR do?

Adds missing deprecation warning.

Context: #6004 (comment)

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is free to review the PR once the tests have passed.
Before you start reviewing make sure you have read Review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR

[codecov]

Codecov Report

Merging #6012 (7708635) into master (f655f97) will increase coverage by 2%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #6012    +/-   ##
=======================================
+ Coverage      91%     93%    +2%     
=======================================
  Files         160     160            
  Lines       11313   11314     +1     
=======================================
+ Hits        10272   10496   +224     
+ Misses       1041     818   -223     

[tchaton]

LGTM !