Deprecate trainer.disable_validation

[carmocca]

What does this PR do?

We have two properties for the same check. Deprecating the least used one.

https://github.com/PyTorchLightning/pytorch-lightning/blob/441e16f61cc2a50f2d725329ee3013a77bbf34e5/pytorch_lightning/trainer/properties.py#L301-L306

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[codecov]

Codecov Report

Merging #8291 (2d44a65) into master (441e16f) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #8291   +/-   ##
======================================
- Coverage      93%     92%   -0%     
======================================
  Files         213     213           
  Lines       13803   13804    +1     
======================================
- Hits        12803   12736   -67     
- Misses       1000    1068   +68     

[awaelchli]

it can also be found in tests/trainer/test_dataloaders, which will produce a warning

[carmocca]

it can also be found in tests/trainer/test_dataloaders, which will produce a warning

That usage has been updated already, grep is not returning any other occurrences. Am I missing something?