fusefs: remove the vfs.fusefs.mmap_enable sysctl

This sysctl was added > 6.5 years ago for no clear reason. Perhaps it was
intended to gate an unstable feature? But now there's no reason to globally
disable mmap. I'm not deleting the -ono_mmap mount option just yet, because
it might be useful as a workaround for bug 237588.

Sponsored by: The FreeBSD Foundation