Remove unused RollbackManager APIs.

RollbackManager.getAvailableRollback and
getPackagesWithAvailableRollback are no longer needed given the
getAvailableRollbacks API. Remove them.

Bug: 112431924
Test: atest RollbackTest
Change-Id: I88a50a0efd97dd5b45c68e0a3e3253f11d0284f6
2 files changed