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.

(cherry picked from commit 02ebe178a2)

units/graphical.target

@@ -13,6 +13,3 @@ After=multi-user.target
 Conflicts=rescue.target
 Wants=display-manager.service
 AllowIsolate=yes
-
-[Install]
-Alias=default.target

units/multi-user.target

@@ -12,6 +12,3 @@ Requires=basic.target
 Conflicts=rescue.service rescue.target
 After=basic.target rescue.service rescue.target
 AllowIsolate=yes
-
-[Install]
-Alias=default.target