Remove WITHOUT_CAPSICUM build support

Capsicum is non-optional as of c24c117b9644 ("Remove
WITHOUT_{CAPSICUM,CASPER} options").

#ifndef WITHOUT_CAPSICUM is left in the source for the benefit of
downstream consumers, but is never defined in FreeBSD.

Reviewed by: oshogbo
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42077