How to fix warning message related to Pantheon module?
Try using module_missing_message_fixer

with drush

you just need to run the following 2 commands 

drush en module_missing_message_fixer
drush mmmff pantheon 


without drush using the user interface


Enable the module
make sure you have the permission Administer Module Missing Message Fixer
Go to: admin/config/system/module-missing-message-fixer