clean up unused distributed sampler logic in trainer

[awaelchli]

What does this PR do?

Follow up to #5743
The kwargs needed for the distributed sampler are provided by the parallel plugins. The unused logic in the trainer is obsolete.
We keep the property in the trainer for backward compatibility and ease of access internally, and reference the property in the training type plugin.

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[codecov]

Codecov Report

Merging #5975 (f1d0405) into master (4f63942) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #5975   +/-   ##
======================================
- Coverage      90%     90%   -0%     
======================================
  Files         170     170           
  Lines       11805   11789   -16     
======================================
- Hits        10674   10640   -34     
- Misses       1131    1149   +18     

[carmocca]

Deleted code is the best code 💃

[tchaton]

Great cleaning !

[Borda]

Deleted code is the best code 💃

https://twitter.com/fishnets88/status/1360865122969923584?s=19