NFSD: Remove do_nfsd_create()

[ Upstream commit 1c388f27 ]

Now that its two callers have their own version-specific instance of
this function, do_nfsd_create() is no longer used.

[ cel: backported to 5.10.y, prior to idmapped mounts ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>