deploy-ng: Remove check for non-empty platforms list in build_apps::finalize_options()

This is actually a supported use-case (we fall back to the Panda3D
platform when no platforms are listed).

direct/src/showutil/dist.py

@@ -123,7 +123,6 @@ class build_apps(distutils.core.Command):
             else:
                 src, _ = path
             assert os.path.exists(src), 'Copy path source does not exist: {}'.format(src)
-        assert self.platforms, 'At least one deploy platform must be defined'
         assert os.path.exists(self.requirements_path), 'Requirements.txt path does not exist: {}'.format(self.requirements_path)
 
     def run(self):