bspatch: Remove backwards-compatibility sys/capability.h support

bspatch previously included sys/capability.h or sys/capsicum.h based
on __FreeBSD_version, as FreeBSD is the upstream for bsdiff and we may
see this file incorporated into other third-party software.

The Capsicum header is now installed as sys/capsicum.h in stable/10 and
FreeBSD 10.3, so we can just use sys/capsicum.h and simplify the logic.

Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D7954