workflows: Remove confusing relic from Ubuntu 18.04

Before commits 97768cea and cb655ab3, we were upgrading GLib to a
post-Ubuntu-18.04 version before running tests, to work around a GLib
bug. We no longer need to do that now that we're running everything on
Ubuntu 20.04 or later.

Fixes: cb655ab3 "check.yml: remove glib260 ppa"
Signed-off-by: Simon McVittie <smcv@collabora.com>

.github/workflows/check.yml

@@ -139,14 +139,6 @@ jobs:
         CFLAGS: -O2 -Wp,-D_FORTIFY_SOURCE=2
     - name: Build flatpak
       run: make -C _build -j $(getconf _NPROCESSORS_ONLN)
-    # We build with Ubuntu 18.04's GLib to prove that we can, but there's a
-    # race condition that makes it fail tests, so upgrade to a version from
-    # a PPA before running the tests: see
-    # https://github.com/flatpak/flatpak/pull/3121,
-    # https://gitlab.gnome.org/GNOME/glib/-/issues/1014
-    - name: Upgrade GLib before running tests
-      run: |
-        sudo apt-get install -y libglib2.0-dev
     - name: Run tests
       run: make -C _build check -j $(getconf _NPROCESSORS_ONLN)
       env: