Delete checkpoint_connector.has_trained

[carmocca]

What does this PR do?

Delete flag added in #4291

Considering this flag internal API even though it was not prefixed by an underscore

Part of #7724

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[codecov]

Codecov Report

Merging #8292 (4e7cdde) into master (441e16f) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #8292   +/-   ##
======================================
- Coverage      93%     92%   -0%     
======================================
  Files         213     213           
  Lines       13803   13800    -3     
======================================
- Hits        12803   12732   -71     
- Misses       1000    1068   +68     

[tchaton]

LGTM !