Remove the deprecated DDP2 strategy

[carmocca]

What does this PR do?

See title

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

Removes deprecated code

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cc @Borda @justusschock @awaelchli @rohitgr7 @akihironitta

[codecov]

Codecov Report

Merging #14026 (8258942) into master (ef0623e) will decrease coverage by 4%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #14026      +/-   ##
==========================================
- Coverage      80%      76%      -4%     
==========================================
  Files         103      323     +220     
  Lines        6843    26505   +19662     
==========================================
+ Hits         5451    20140   +14689     
- Misses       1392     6365    +4973