linux(4): Get rid of Linuxulator kernel build options.

Stop confusing people, retire COMPAT_LINUX and COMPAT_LINUX32 kernel
build options. Since we have 32 and 64 bit Linux emulators, we can't build both
emulators together into the kernel. I don't think it matters, Linux emulation
depends on loadable modules (via rc).

Cut LINPROCFS and LINSYSFS for consistency.

PR: 215061
Reviewed by: bcr (manpages), trasz
Differential Revision: https://reviews.freebsd.org/D30751
MFC after: 2 weeks