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>
Git-commit: 590a95e418d18894e34cd240b3e7ef278add05c7
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/


[lixiang@codeaurora.org: resolve trivial merge conflicts]
Change-Id: I24f4da72efcbde084f917a2be75dcc5e5c1f711c
Signed-off-by: lixiang <lixiang@codeaurora.org>