Remove deprecated distributed_backend from Trainer

[rohitgr7]

What does this PR do?

Follow up: #9246
Remove deprecated distributed backend.

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

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[codecov]

Codecov Report

Merging #10017 (328f9e5) into master (8f14e77) will decrease coverage by 0%.
The diff coverage is 100%.

❗ Current head 328f9e5 differs from pull request most recent head c26ae88. Consider uploading reports for the commit c26ae88 to get more accurate results

@@           Coverage Diff           @@
##           master   #10017   +/-   ##
=======================================
- Coverage      93%      93%   -0%     
=======================================
  Files         179      179           
  Lines       15810    15812    +2     
=======================================
- Hits        14635    14634    -1     
- Misses       1175     1178    +3     

[awaelchli]

excellent!

[kaushikb11]

LGTM! Thanks