Cleanup remnants of dynamic_system feature flag.
This piece of code only makes sense when we had the feature flag.
Now that the feature flag is deprecated, we shouldn't check the
DynamicSystemManager.isInUse() anymore, because we actually do want to
post a notification when DSU is installed but not in use.

Bug: 176795908
Test: Install a DSU, reboot into guest system, reboot back to host
  system, verify the BootCompletedReceiver is active.
Change-Id: I31c438fcd70e71eb2a5b14cdb713bd25516e8532