Remove the deprecated weights_save_path Trainer argument

[awaelchli]

What does this PR do?

Removes

• Trainer(weights_save_path=...) argument
• Trainer.weights_save_path property

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @tchaton @rohitgr7 @justusschock @akihironitta

[Felonious-Spellfire]

LGTM

[tchaton]

LGTM !