Remove write_predictions from LightningModule

[ananthsub]

What does this PR do?

write_predictions was marked as deprecated in v1.3 and is being removed in v1.5. Users can write outputs directly from their lightning module to their preferred destination, or use the prediction writer callbacks now offered.

This contributes to #7315 as better hygiene and separation across components.

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

Remove write_predictions and write_predictions_dict from LightningModule.

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[codecov]

Codecov Report

Merging #8850 (482b32e) into master (e060547) will increase coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #8850   +/-   ##
======================================
  Coverage      89%     89%           
======================================
  Files         172     172           
  Lines       14178   14114   -64     
======================================
- Hits        12568   12512   -56     
+ Misses       1610    1602    -8     

[tchaton]

LGTM !