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

(cherry picked from commit b658c0fce10a06493dc468fc9f6699120473e17b)
(cherry picked from commit ece63e96c364679e19c124d145abe5af789d1948)