TrustManagerService: remove ENABLE_ACTIVE_UNLOCK_FLAG

The system property fw.enable_active_unlock_flag defaults to true, and I
can't find any evidence that anyone is setting it.  Therefore, the code
that handles the false case seems to be dead code.  Remove this code.

Bug: 215724686
Test: atest TrustTests
Change-Id: I57de36bade75c457dc7f86225817899f70dc4e48