ip_mroute: Delete unreachable code

As the flag M_WAITOK is passed to ip_encap_attach(), then the function
will never return NULL, and the following code within NULL check branch
will be unreachable.

No functional change intended.

Reviewed by: kp
Fixes: 6d8fdfa9d5e7d Rework IP encapsulation handling code
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39746