net: remove legacy in_addmulti()

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).

Reviewed by: bz@, melifaro@
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D28918