Update, Stein Åsmul: Injecting this newer list of cleanup approaches.
Find your package in
C:\Windows\Installer, where Windows keeps copies of installed MSI packages. The names are generated randomly, so you’ll have to look at the creation dates of the files.
Open the MSI file with Orca. (Unfortunately there is no simple download for the orca installer. You can get it by installing the “MSI Tools” of the Windows 10 SDK, and then searching for orca.msi in
C:\Program Files (x86)\Windows Kits.)
Delete the offending custom action from the CustomAction table
Now you should be able to uninstall the package.
UPDATE: You can find the actual cache MSI file using Powershell. That was for one package, you can also get for all packages (scroll down to first screenshot).