remove support for optimizer_idx in the training_step for manual optimization

[awaelchli]

What does this PR do?

This was deprecated and is not any longer valid:

def training_step(self, batch, batch_idx, optimizer_idx)
   ...

This PR converts the warning to an error.

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[pep8speaks]

Hello @awaelchli! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-07-29 07:50:35 UTC

[codecov]

Codecov Report

Merging #8576 (3f31879) into master (0dc0472) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #8576    +/-   ##
=======================================
- Coverage      93%     88%    -4%     
=======================================
  Files         168     168            
  Lines       13951   13951            
=======================================
- Hits        12909   12315   -594     
- Misses       1042    1636   +594