If you don't have a /etc/apt/sources.list.d/qgis.sources file yet, you can simply create it.
Either use an editor like nano, mousepad, geany etc to edit the file directly:
sudo YOUREDITOR /etc/apt/sources.list.d/qgis.sources
or create a qgis.sources text file with your normal user using any text editor and then move it using sudo mv qgis.sources /etc/apt/sources.list.d/
Purging a package using apt is done by apt purge packagename, so e.g. apt purge qgis.