Remove VirtualBox guest shared folders filesystem

This went into staging in rc7.  It turns out that was a mistake, and
apparently it wasn't even supposed to go there at all, but be introduced
as a regular filesystem.

We don't try to sneak in whole new filesystems this late in the rc, just
delete the whole thing, and it can be re-introduced as a proper patch
with proper acks from actual filesystem people instead of some odd
late-rc staging back-door.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>