staging: exfat: remove staging version of exfat filesystem

Now that there is a "real" solution for exfat in the vfs tree queued up
to be merged in 5.7-rc1 the "old" exfat code in staging can be removed.

Many thanks to Valdis for doing the work to get this into the tree in
the first place, it was greatly appreciated.

Acked-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Cc: Pali Rohár <pali@kernel.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Namjae Jeon <namjae.jeon@samsung.com>
Cc: Sungjong Seo <sj1557.seo@samsung.com>
Cc: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20200310105421.GA2810679@kroah.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>