Remove deprecated LoggingContext.set_current_context/LoggingContext.current_context methods

[MadLittleMods]

Remove deprecated LoggingContext.set_current_context/LoggingContext.current_context methods

These were added for backwards compatibility (and essentially deprecated) in matrix-org/synapse#7408 (2020-05-04) because synapse-s3-storage-provider previously relied on them -- but synapse-s3-storage-provider since been updated to no longer use them.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
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• Code style is correct (run the linters)

[MadLittleMods]

Thanks for the review @anoadragon453 🐕