Remove truncated_bptt_steps from Trainer constructor

[ananthsub]

What does this PR do?

This flag was deprecated in v1.3 and is being removed in v1.5
The property has been moved to the LightningModule under the same property name
This is a follow up to comments in #8804
Rather than validating that the trainer and lightning module values were the same if both were set, it's easier to remove the trainer settings altogether

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

Breaking change: Removed truncated_bptt_steps from Trainer constructor

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[codecov]

Codecov Report

Merging #8825 (779e182) into master (c4a1c8b) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #8825   +/-   ##
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- Coverage      89%     89%   -0%     
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  Files         169     169           
  Lines       14069   14061    -8     
======================================
- Hits        12463   12455    -8     
  Misses       1606    1606