cfumass(4) is not usable if usfs(4) is loaded or compiled into the
kernel. Remove usfs so that the user may kldload the USB mass storage
target they prefer.
PR: 218169
Reviewed by: trasz, hselasky (no objection)
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D10153