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

uefisign previously included sys/capability.h or sys/capsicum.h based
on __FreeBSD_version in order to facilitate development on the stable
branch. The Capsicum header is now installed as sys/capsicum.h in
stable/10 and FreeBSD 10.3, so there's no need for the backwards
compatibility support.

Reviewed by: trasz
Sponsored by: The FreeBSD Foundation