Removing receiver enforcement flag.

It's time to permanently turn on enforcement for registering receivers
Dynamically. This change will remove the flag that controls whether
enforcement can be turned off, but this is strictly a no-op since the
default value is already "on".

BUG: 234659204
Test: no-op
Change-Id: I14dd99144abc58afa49855b96ba67d0ab84bc502