Remove BackupManagerServiceInterface

Interface was created for switching between refactored and
non-refactored version, not needed anymore. Removed with IntelliJ's
Refactor > Inline... tool.

Bug: 22388012
Test: atest FrameworksServicesTests
Test: atest FrameworksServicesRoboTests
Test: adb shell bmgr backupnow <package>
Change-Id: I7d92b73676634ddd09a1bc71cbcfc4400ad1be90