remove metric deprecation message at module import level

[awaelchli]

What does this PR do?

Every Lightning user who imports PL, whether they want torchmetrics or not, are getting this warning.
It makes no sense to print a warning and import it ourselves one level higher up in pytorch_lightning/__init__.py

All functions in the metrics package are already decorated with a deprecation warning message.

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Did you have fun?

I made sure I had fun coding 🙃

[codecov]

Codecov Report

Merging #8163 (99a95ab) into master (c3065c5) will decrease coverage by 5%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #8163    +/-   ##
=======================================
- Coverage      93%     88%    -5%     
=======================================
  Files         211     211            
  Lines       13440   13438     -2     
=======================================
- Hits        12450   11829   -621     
- Misses        990    1609   +619     

[ethanwharris]

Yay 😃