Remove pytorch lightning.callbacks.lr monitor.learning rate monitor.lr_sch_names

[rschireman]

What does this PR do?

Fixes part of #12521 by removing the deprecated pytorch lightning.callbacks.lr monitor.learning rate monitor.lr_sch_names

resolves #10066

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

Yes, removes pytorch lightning.callbacks.lr monitor.learning rate monitor.lr_sch_names`

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[rschireman]

@akihironitta

[akihironitta]

Hi @rschireman, thanks for working on this!

Could you add a changelog entry for both add_lr_sch_names and lr_sch_names?
(UPDATE: an entry for add_lr_sch_names is actually unnecessary as it's not part of the public API. Sorry for the confusion.)

Once it's added, let's mark this PR as ready for review and see all the existing tests pass in our CI :]