The only package which is pre-installed on your system is foomatic-db-compressed-ppds. ubuntu-desktop depends on foomatic-db-compressed-ppds, but the removal of foomatic-db-compressed-ppds, which also removes ubuntu-desktop, doens't destroys your system, as ubuntu-desktop is only a meta-package.
This package contains all PPD files which can be generated from the
Foomatic database as one small compressed archive. This reduces the
disk space consumption compared to the Foomatic XML database vastly
and in addition makes listing of all available PPDs and extracting the
needed PPD much faster than using the Foomatic XML database.
Both packages, cups-pdf and printer-driver-cups-pdf, don't depends on foomatic-db-compressed-ppds.
On the other side, if you need any foomatic package later by a dependency, let's resolve this problem later, if there is any.