Remove the -C option as it does more harm than good. To be fully
compatible, it would have to (at least):

• support the "compat-compat" -T option,
• *not* support the -l, -O, and -v options,
• default to soft updates being disabled.

Worse, the compatibility mode makes it impossible to mount_mfs(8)
a file system from fstab(5) with soft updates disabled (-S). [1]

Now, the only difference when called as "mount_mfs" or "mfs" (as
opposed to "mdmfs") is that the file mode of the mount point is
set by default to 01777. All options available to mdmfs(8) are
also available to mount_mfs(8); the -C option is still recognized
but ignored for backward compatibility.

PR: bin/98860 [1]
MFC after: 2 weeks