[NETFILTER]: x_tables: remove duplicate of xt_prefix

Remove xt_proto_prefix array which duplicates xt_prefix and change all
users of xt_proto_prefix to xt_prefix.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>