net: remove legacy in_addmulti()

[kp]

Summary

Despite the comment to the contrary neither pf nor carp use
in_addmulti(). Nothing does, so get rid of it.

Carp stopped using it in 08b68b0e4c6b132127919cfbaf7275c727ca7843
(2011). It's unclear when pf stopped using it, but before
d6d3f01e0a3395c1fae34a3c4be7b051cb2d7581 (2012).

[bz]

Looks ok to me; Do we assume that no kmods from port would still use this?

[kp]

That would be my assumption.

It's hard to know for sure no one uses this. The best way to figure that out is to remove it and wait for the shouting to start.
I'd say we should remove it in main (i.e. 14), but not MFC the removal. That should give us plenty of time to notice if this does break something.

[melifaro]

+1 for removing and not MFCin.