Remove deprecated LoggerCollection

[awaelchli]

What does this PR do?

Follow up to #14231
Resolves #12147

This also reverts #10232, which is no longer needed now that LoggerCollection is gone and user is forced to use self.loggers to access all logger instances.

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Did you have fun?

I made sure I had fun coding 🙃

cc @awaelchli @edward-io @Borda @ananthsub @rohitgr7 @kamil-kaczmarek @Raalsky @Blaizzy @tchaton @justusschock

[tchaton]

LGTM !