msdosfs: mark unused compat-mount fields

The magic number MSDOSFS_ARGSMAGIC, which used to distinguish
"old" vs "new" msdosfs mount arguments, has not been used since
2005; it should just go away now.

Likewise, the local-to-Unicode table that changed at the same
time is unused.

Leave the space reserved in the old style mount arguments, though,
since we still support the old mount call (via the cmount entry
point).

Submitted by: Chris Torek <chris.torek@gmail.com>
MFC after: 2 weeks