Remove should_rank_save_checkpoint property from Trainer

[kaushikb11]

What does this PR do?

Fixes #9074

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[ananthsub]

@kaushikb11 we can also mark this as deprecated or remove this: https://github.com/PyTorchLightning/pytorch-lightning/blob/0d3325ea206d429f65a52e9d818751a168ec7780/pytorch_lightning/plugins/training_type/training_type_plugin.py#L311-L314

[codecov]

Codecov Report

Merging #9433 (a7525fa) into master (0f8fd20) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #9433    +/-   ##
=======================================
- Coverage      93%     89%    -4%     
=======================================
  Files         178     178            
  Lines       15653   15650     -3     
=======================================
- Hits        14508   13899   -609     
- Misses       1145    1751   +606