cifs: Remove cifs_aio_ctx

Remove struct cifs_aio_ctx and its associated alloc/release functions as it
is no longer used, the functions being taken over by netfslib.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Steve French <sfrench@samba.org>
cc: linux-cifs@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>