ci: Rely on the monorepo's gtk subproject support

The monorepo now ensures that the gtk subproject keeps building on
windows and macOS via its own CI.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1499>

ci/install-gst.sh

@@ -24,11 +24,11 @@ PLUGINS="-D gst-plugins-base:ogg=enabled \
          -D gst-plugins-bad:opus=enabled \
          -D gst-plugins-ugly:x264=enabled"
 
-echo "subproject('gtk')" >> meson.build
 meson setup build \
     -D prefix=/usr/local \
     -D gpl=enabled \
     -D ugly=enabled \
+    -D gtk=enabled \
     -D examples=disabled \
     -D gtk_doc=disabled \
     -D introspection=disabled \

ci/windows-docker/install_gst.ps1

@@ -38,14 +38,12 @@ $MESON_ARGS = @(`
   "-Dpython=disabled", `
   "-Dlibav=disabled", `
   "-Dvaapi=disabled", `
+  "-Dgtk=enabled", `
   "-Dgst-plugins-base:pango=enabled", `
   "-Dgst-plugins-good:cairo=enabled", `
   "-Dgst-plugins-good:lame=disabled"
 )
 
-$PSDefaultParameterValues['Out-File:Encoding'] = 'utf8'
-echo "subproject('gtk')" >> meson.build
-
 Write-Output "Building gstreamer"
 meson setup --vsenv $MESON_ARGS _build
 if (!$?) {