removing memchache mentions in comments

remove memcached folder as it is no longer needed, also address a couple grammatical errors

awx/main/management/commands/regenerate_secret_key.py

@@ -82,7 +82,7 @@ class Command(BaseCommand):
             OAuth2Application.objects.filter(pk=app.pk).update(client_secret=encrypted)
 
     def _settings(self):
-        # don't update memcached, the *actual* value isn't changing
+        # don't update the cache, the *actual* value isn't changing
         post_save.disconnect(on_post_save_setting, sender=Setting)
         for setting in Setting.objects.filter().order_by('pk'):
             if settings_registry.is_setting_encrypted(setting.key):

awx/main/management/commands/run_dispatcher.py

@@ -44,7 +44,7 @@ class Command(BaseCommand):
 
         # It's important to close these because we're _about_ to fork, and we
         # don't want the forked processes to inherit the open sockets
-        # for the DB and memcached connections (that way lies race conditions)
+        # for the DB and cache connections (that way lies race conditions)
         django_connection.close()
         django_cache.close()