fusefs: delete some unused mount options

The fusefs kernel module allegedly supported no_attrcache, no_readahed,
no_datacache, no_namecache, and no_mmap mount options, but the mount_fusefs
binary never did. So there was no way to ever activate these options.
Delete them. Some of them have alternatives:

no_attrcache: set the attr_valid time to 0 in FUSE_LOOKUP and FUSE_GETATTR
responses.
no_readahed: set max_readahead to 0 in the FUSE_INIT response.
no_datacache: set the vfs.fusefs.data_cache_mode sysctl to 0, or (coming
soon) set the attr_valid time to 0 and set FUSE_AUTO_INVAL_DATA in
the FUSE_INIT response.
no_namecache: set entry_valid time to 0 in FUSE_LOOKUP and FUSE_GETATTR
responses.

Sponsored by: The FreeBSD Foundation