Bluetooth: Remove rx < tx prio check

The comment states the reason for this requirement is to detect
violations by a remote peer. The ATT version of this was removed in
ea04fd95. TLDR: It's neither possible
for, nor the job of the host to police the remote device.

We remove this requirement to be more flexible about the number of
priority levels in the system, and to avoid the temptation of using
priorities as a synchronization mechanism.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>