cifs: remove set but not used variable 'sep'

Fixes gcc '-Wunused-but-set-variable' warning:

fs/cifs/cifs_dfs_ref.c: In function 'cifs_dfs_do_automount':
fs/cifs/cifs_dfs_ref.c:309:7: warning:
 variable 'sep' set but not used [-Wunused-but-set-variable]

It never used since introdution in commit 0f56b277073c ("cifs: Make use
of DFS cache to get new DFS referrals")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Paulo Alcantara <palcantara@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>