CI: remove workaround for some subproject builds failing on Windows

CI: remove workaround for some subproject builds failing on Windows

In some cases subproject builds under MSYS2 failed, see
https://gitlab.gnome.org/GNOME/gtk/-/issues/2243
https://gitlab.gnome.org/GNOME/gtk/-/issues/3002
https://gitlab.gnome.org/GNOME/gtk/-/issues/5053

These manual pango/glib build instructions were added to work around
the problem by avoiding the subprojects.

https://gitlab.gnome.org/GNOME/gtk/-/issues/5053 uncovered the bug in
binutils, which is now fixed in master there and was also backported
into MSYS2 which we use in CI.

So we can just remove the workaround again now.