chore: remove old script code for ATOM_SHELL_ (#37140)

The TODO is from 2016, seems like it's time

script/lib/config.py

@@ -31,14 +31,7 @@ def get_target_arch():
 
 
 def get_env_var(name):
-  value = os.environ.get('ELECTRON_' + name, '')
-  if not value:
-    # TODO Remove ATOM_SHELL_* fallback values
-    value = os.environ.get('ATOM_SHELL_' + name, '')
-    if value:
-      print('Warning: Use $ELECTRON_' + name +
-            ' instead of $ATOM_SHELL_' + name)
-  return value
+  return os.environ.get('ELECTRON_' + name, '')
 
 
 def enable_verbose_mode():