Speed up tests

Don't reboot the device when there is no file to delete. This saves us a
significant amount of time since each reboot takes 1 min and there are
dozens of reboot during the whole test case.

Bug: 149528114
Test: atest StagedRollbackTest

Change-Id: If3b06a304d9c87680c7cc5b7498d3b2ecf11cdfa