Finish deprecating legacy MediaScanner.

We switched completely over to using ModernMediaScanner in Android Q,
and this legacy code is no longer being used.  Clean it up, but leave
enough remnants behind to begrudgingly keep reflecting apps working.

Bug: 128243527
Test: atest --test-mapping packages/providers/MediaProvider
Change-Id: Ib2bc341fafcd4c2e049c864a35fbe651d013f6fd