Remove usfs(4) from arm kernel configs

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