Drop support for full backup of com.android.providers.settings

The only reason it was still there was adb backup. It's no longer
needed since we support adb backup K/V since Oreo.
Full restore code has to stay for backwards compatibility, but
will not need to be made forward-compatible, since no backups
will be made on P+.

Test: adb backup com.android.providers.settings
Bug: 71746093
Change-Id: I2e8f90d57ed73bb3e050c0e345afef4ad2d3fb82