To answer your main question first - for getting rid of the sudo apt-get update error message, you have to remove the Google Chrome repository - open a terminal and execute this command :
sudo rm /etc/apt/sources.list.d/google-chrome.list
To completely remove Chromium and Google Chrome execute :
sudo apt-get purge chromium-browser google-chrome-stable
When you also want to remove the configuration files, delete these folders :
/home/user/.config/chromium and /home/user/.config/google-chrome.
Note : I assume that you have the google-chrome-stable package installed.
Update :
I received an information that there might be another Google .list file in the sources.list.d folder - so in case the error still occurs, check this folder and correct or delete the entries as well.