Clean up media_load_metadata_via_media_data_loader

Flag has been launched - remove code paths that are no longer used, plus
some tests that are redundant with coverage in MediaDataLoaderTest

Flag: EXEMPT removing  com.android.systemui.media_load_metadata_via_media_data_loader
Bug: 358350077
Test: LegacyMediaDataManagerImplTest
Test: MediaDataProcessorTest
Change-Id: I5fe896e9a71e99db2b91b561dff59c8ecfd51130
5 files changed