Cleanup mcast_join_leave function in PlainDatagramSocketImpl

Removed codepaths responsible for handling MACOSX and 2.2 linux kernel.
This method is quite convoluted, this de-cluttering makes it easier to
understand what it's doing.

Bug: 25993730
Change-Id: Ibdb8a969fd4e3869fb307df6d83dc600ed3ebd35
1 file changed