nfsd: remove unused get_new_stid()

get_new_stid() is no longer used since commit 3abdb607 ("nfsd4:
simplify idr allocation").  Remove it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>