Stop building resource PWADs and "make dist" generates a single Zip

The resource PWADs have been around for a very long time and dated
from before Freedoom was capable of running as a complete IWAD on its
own. Few people ever cared about these files, as evident by a lack of
reports about freedoom_levels.wad not even being correct.

Additionally, the separate Zips for each IWAD is gone. With that,
Freedoom will be distributed as a single Zip file only that includes
all of the subprojects.