Removal/1.5 lightning module datamodule property

[Tshimanga]

What does this PR do?

Removal of datamodule property from LightningModule as scheduled for 1.5

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

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[codecov]

Codecov Report

Merging #9233 (5ee1c63) into master (ccc83e7) will decrease coverage by 4%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #9233    +/-   ##
=======================================
- Coverage      92%     88%    -4%     
=======================================
  Files         176     176            
  Lines       14810   14801     -9     
=======================================
- Hits        13662   13042   -620     
- Misses       1148    1759   +611     

[carmocca]

heads up for @ethanwharris as I believe Flash relies on this

[tchaton]

LGTM !

[awaelchli]

maybe @Tshimanga wants to make a PR to flash as well 😃

[Tshimanga]

maybe @Tshimanga wants to make a PR to flash as well smiley

@awaelchli @ethanwharris yeah I can take care of that. Probably won't be able to get around to it till Tues (9/7)