IPoIB: Remove unused IPOIB_MCAST_STARTED code

The IPOIB_MCAST_STARTED flag is not used at all since commit b3e2749b


("IPoIB: Don't drop multicast sends when they can be queued"), so
remove it.

Signed-off-by: Eli Cohen <eli@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>