Remove errors no longer reported. Since PHP 7.x is no longer supported, remove
the 7.x baseline. It is now no longer necessary to have multiple baseline
files.
phpstan is not run with an action anymore, as the action runs
it its own docker container and hence has no access to files
outside the repository root. A side-effect of this is, that
phpstan now **really** runs with the php version set up by
the matrix.
