build system does not try to remove mutt anymore since 3.12~rc1

02697edc52edb9d4cecc796135014dcca7aa0f3d uses package_is_installed, which does not exist, so:

/root/98-remove_unwanted_packages: 51: /root/98-remove_unwanted_packages: package_is_installed: not found

… then even if mutt was installed, we will never run the command that removes it.

I guess that using the correct function name (is_package_installed) should be enough to fix it.

Original created by @intrigeri on 16392 (Redmine)