crashinfo: remove unused /usr/bin/gdb support

We have not had gdb 6.1 in the base system for some time; there is no
need to check for it.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34478

(cherry picked from commit 13a974574670cc85a7db60bae6620631ad756f2a)