remove deprecation of gpu string parsing behavior

[awaelchli]

What does this PR do?

I introduced this change in behavior in 1.2.
The deprecation window is now over and we parse strings for --gpus N as the number of devices to use and no longer the index of GPU to use.

To emphasize, this inconsistency only applied when gpus was a string.

Does your PR introduce any breaking changes? If yes, please list them.

YES.

Before 1.5:
trainer = Trainer(gpus="2") selects the 3rd gpu (index 2)
Now:
trainer = Trainer(gpus="2") selects the two first available GPUs (0, 1) and is consistent with gpus=int.

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[codecov]

Codecov Report

Merging #8770 (cea12d4) into master (69f287e) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #8770   +/-   ##
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- Coverage      93%     93%   -0%     
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  Files         169     169           
  Lines       14077   14088   +11     
======================================
+ Hits        13045   13055   +10     
- Misses       1032    1033    +1