build: do not build GLib for Windows anymore.

We were building it to add the patch glib!2020, but it has now been
backported in MSYS2 package:
msys2/MINGW-packages#9154
This was about the most infamous bug #913 for very slow file dialogs on
Windows when some drives are disconnected, or with slow/non-accessible
network drives or even fake floppy drives created in the Bios.

Similarly we also wanted glib!2205 and glib!2210 for bug #6780 about
GIMP crashing unexpectedly when images are opened in other (apparently
unrelated applications). I had not updated our build scripts yet, but
anyway, it got backport to the MSYS2 package first, then even to GLib
2.68.4 which has been recently released (and bumped in MSYS2 as well).
See: msys2/MINGW-packages#9283

So let's rely again on MSYS2 package!

build/windows/gitlab-ci/build-deps-msys2.sh

@@ -71,38 +71,6 @@ make
 make install
 cd ../..
 
-## GLib (patched from MSYS2) ##
-
-git clone --branch 2.68.0 --depth=${GIT_DEPTH} https://gitlab.gnome.org/GNOME/glib.git _glib
-
-cd _glib/
-wget "https://raw.githubusercontent.com/msys2/MINGW-packages/master/mingw-w64-glib2/0001-Update-g_fopen-g_open-and-g_creat-to-open-with-FILE_.patch"
-wget "https://raw.githubusercontent.com/msys2/MINGW-packages/master/mingw-w64-glib2/0001-disable-some-tests-when-static.patch"
-wget "https://raw.githubusercontent.com/msys2/MINGW-packages/master/mingw-w64-glib2/0001-win32-Make-the-static-build-work-with-MinGW-when-pos.patch"
-wget "https://raw.githubusercontent.com/msys2/MINGW-packages/master/mingw-w64-glib2/0002-disable_glib_compile_schemas_warning.patch"
-git apply 0001-Update-g_fopen-g_open-and-g_creat-to-open-with-FILE_.patch
-git apply 0001-win32-Make-the-static-build-work-with-MinGW-when-pos.patch
-patch -p1 < 0001-disable-some-tests-when-static.patch
-git apply 0002-disable_glib_compile_schemas_warning.patch
-# Only this patch is different from MSYS2 build.
-git apply ../build/windows/patches/glib-mr2020.patch
-
-mkdir _build
-cd _build
-meson -Dprefix="${GIMP_PREFIX}" -Dlibelf=disabled --buildtype=release \
-      --wrap-mode=nodownload --auto-features=enabled \
-      -Ddefault_library=shared -Dforce_posix_threads=true -Dgtk_doc=false ..
-ninja
-ninja install
-cd ../..
-
-# glib-networking is needed. No need to rebuild it, since we build the
-# same version of glib with the same options, and just some additional
-# patches, so we assume MSYS2-built packages should be fine.
-mkdir -p ${GIMP_PREFIX}/lib/gio/modules/
-cp -fr ${MSYS_PREFIX}/lib/gio/modules/*.dll ${GIMP_PREFIX}/lib/gio/modules/
-# TODO: what about /mingw64/share/locale/*/LC_MESSAGES/glib-networking.mo ?
-
 ## GTK (patched from MSYS2) ##
 
 pacman --noconfirm -S --needed \