ci: Clean up gobject-introspection build artifacts

I don’t see there’s any reason for us to keep and expose artifacts from
building gobject-introspection as a dependency. The introspection files
subsequently generated for GLib are/can be exposed separately as
artifacts.

In particular, keeping the gobject-introspection directory around as a
subdirectory of the glib.git checkout means that `reuse.sh` starts
investigating the licenses of the gobject-introspection.git files and
failing as a result.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

.gitlab-ci.yml

@@ -154,11 +154,8 @@ variables:
         git clone --branch "${GOBJECT_INTROSPECTION_TAG}" https://gitlab.gnome.org/GNOME/gobject-introspection.git gobject-introspection
         meson setup gobject-introspection gobject-introspection/build --prefix=/usr
         sudo meson install -C gobject-introspection/build
+        rm -rf gobject-introspection
       fi
-  artifacts:
-    expire_in: 3 days
-    paths:
-      - gobject-introspection
 
 .build-linux:
   before_script: