Remove sys/capability.h for the third time

In all supported (and most unsupported) FreeBSD versions the appropriate
header for Capsicum is sys/capsicum.h. Software including sys/capability.h
is most likely looking for Linux capabilities based on the withdrawn
POSIX.1e draft.

This header was previously removed in r334929 and r340156, but reverted
each time due to ports failures. These issues have now (broadly) been
addressed.

PR: 228878 [exp-run]
Submitted by: eadler (r334929)
Relnotes: Yes
Sponsored by: The FreeBSD Foundation