igmp: remove unused macro IGMP_Vx_UNSOLICITED_REPORT_INTERVAL

After commit 2690048c ("net: igmp: Allow user-space
configuration of igmp unsolicited report interval"), they
are not used now

Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>