units: drop [Install] section from multi-user.target and graphical.target

They were supposed to make it easy to make the default.target a symlink
to these targets, but this was never advertised and we have a better
command for this now in "systemctl set-default". Since the install
section makes the output of "systemctl list-unit-files" confusing (since
it makes the units appear as "disabled"), let's drop the sections.