remove deprecated model_size from LightningModule

[tkonopka]

What does this PR do?

Part of #12521

resolves #8343

Primary change:

• removed the deprecated model_size property/method from the LightningModule class

Associated edits:

• removed "model_size" from an internal list of __jit_unused_properties__
• removed an import made redundant by the above edits
• removed a unit test associated with the removed method
• removed a few lines of documentation describing the removed method
• updated the changelog

Notes:

Following a suggestion from @kaushikb11 in one of the comments in #12521, a search for ".model_size" on grep.app gave 438 hits. This pattern is quite generic, so it is not surprising there are many hits not related to lightning or to this PR. I browsed through them, quickly omitting cases attempting to assign a model size rather than retrieve it and cases where a model size was compared to strings. I investigated the others for signs of importing lightning or including lightning in the repo requirements file. The only proper hit I found was a repository intel-analytics/BigDL. The authors seem to be already aware that the model_size property is deprecated (see here), so no further action taken.

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

• removed property model_size in the LightningModule class

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