icmp: drop unused MIB accounting wrappers

There are ICMP_XXX_STATS that are not used in the kernel, so I remove
them, not to "just patch" them later. But if there's some sense in
keeping them, kick me - I will remake this set keeping them.

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