Launchpad data is stored in a SQLite 3 database located at:
Whether or not you'll be able to successfully edit it depends on your SQL skills.
You could always just delete the file, probably best from Recovery HD, and it will rebuild itself when logging back in and opening Launchpad. You could also just try deleting it and then in Terminal run the
killall Dock command, then open Launchpad. It should rebuild with only the currently installed apps.
Another thing to consider is maybe rebuilding the Launch Services Database may help. Note: Rebuilding the Launch Services Database may remove some customizations you've made and would need to set them once again.
In a Terminal:
/S*/L*/F*/C*/F*/L*/S*/lsregister -kill -r -domain local -domain system -domain user
Hint: Copy and paste the command above in the Terminal to avoid typos.
The full pathname is:
Although you can use the wildcard version, you don't need the fully qualified pathname version for it to execute.