Remove all traces of "srb2.wad" support, since it is no longer needed now that...

Remove all traces of "srb2.wad" support, since it is no longer needed now that srb2.pk3 is the IWAD (...IPK3?). srb2.wad cannot exactly be a pk3 file after all.

Additionally, rename any remaining "srb2.srb" to "srb2.pk3" in the main source code files. Not sure whether to bother dealing with srb2.srb/srb2.wad mentions in project files for now.