Remove tests (13/n)

Remove tests that have been migrated to CTS-root/CTS.

* RollbackTest#testMultipleRollbackAvailable is removed.
  The test is no longer needed as the API it tests
  (RollbackManager#getAvailableRollback(String packageName))
  is removed by ag/6153954.

Bug: 194971428
Test: m StagedRollbackTest RollbackTest
Change-Id: I044f24427c94e2037defe203655c1f8ec25fa6c6