Revert "Support serialized checkpoint loading"

[jjenniferdai]

Reverts #9605

temp revert as I think about how to best fix this with #10061 :
#9605 with #9413 causes serialized loading TTPs' validate/test/predict() to use resume_from_checkpoint instead of ckpt_path:

1. Fix restoring training state during trainer.fit only #9413 adds resume_start call for all states instead of fit only, after trainer._load_checkpoint_weights() call with ckpt_path
2. resume_start with resume_from_checkpoint path --> Support serialized checkpoint loading #9605 serialized TTP load_checkpoint loads model state instead of loading ckpt file only

[codecov]

Codecov Report

Merging #10057 (0e6a176) into master (5b30a65) will decrease coverage by 0%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #10057   +/-   ##
=======================================
- Coverage      93%      93%   -0%     
=======================================
  Files         180      180           
  Lines       15908    15889   -19     
=======================================
- Hits        14722    14703   -19     
  Misses       1186     1186           

[tchaton]

LGTM !