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.