Remove i_net.c

The code in i_net.c doesn't actually seem to be used in SRB2. I was able to compile a build without it, and hosting and joining netgames worked just fine (well, as fine as they can with the current state of the netcode...).

The vast majority of code in the file seems to be contained in HAVE_SDLNET ifdefs, and I'm pretty sure SRB2 has never used SDLNET in a public build. The only bit not contained in that block is I_InitNetwork(), which just prints an error and returns false.

Do we really need to keep it around? If not, I say get rid of it. It seems like useless clutter that is just going to confuse people who are trying to understand the source code.