Do not adjust interface MTU automatically. Leave this task to the system
administrator.

Before r274246 interface MTU was adjusted only when GRE key is configured.
The r274246 has changed this behavior to automatically adjust MTU when any
option, that changes the size of GRE header is configured.
This patch removes automatic MTU adjustment from if_gre(4) and if_me(4),
and restores the behavior that was prior to r274246.

Differential Revision:	https://reviews.freebsd.org/D10215