Revert "mcast: fix memory leak in imf_purge()"

This reverts commit fa03d37432caf17d56a931a9e6f5d9b06f102c5b.

This commit caused us to not send IGMP leave messages if the inpcb went
away. In other words: we freed pending packets whenever the socket
closed rather than when the interface (or address) goes away.

Reviewed by: glebius
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D43032