Deprecate prefix argument in ModelCheckpoint

[rohitgr7]

What does this PR do?

Deprecated prefix argument in ModelCheckpoint. It's no longer needed since we have filename now in ModelCheckpoint.
#4557 (comment)

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[codecov]

Codecov Report

Merging #4765 (a26c9d8) into master (69b9949) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #4765   +/-   ##
======================================
  Coverage      93%     93%           
======================================
  Files         117     117           
  Lines        8974    8977    +3     
======================================
+ Hits         8347    8350    +3     
  Misses        627     627           

[gdhdgy]

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