Revert "MountService: Account for switchable vold pair"

This reverts commit 0a0fe89f5a5b898de531f0ed41e9c7fd0e4f7e08.
Revert "Allow mounting of multiple volumes via mass storage (framework part)"

This reverts commit 850f5577344e9702c2cb49435230f4d942f8d5ca.