Remove Trainer._strategy_type

[carmocca]

What does this PR do?

Part of #11449

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

Removes the protected _strategy_type.

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Did you have fun?

Make sure you had fun coding 🙃

cc @Borda @justusschock @awaelchli @rohitgr7 @akihironitta

[tchaton]

LGTM !