[SNMP]: Remove unused devconf macros.

The SNMP_INC_STATS_OFFSET_BH is used only by ICMP6_INC_STATS_OFFSET_BH.
The ICMP6_INC_STATS_OFFSET_BH is unused.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>