gtk: nuke the legacy apprt from orbit

We don't really have any large outstanding regressions on -ng to warrant
keeping this alive anymore. ¡Adiós!

.github/workflows/test.yml

@@ -22,7 +22,6 @@ jobs:
       - build-macos-matrix
       - build-windows
       - test
-      - test-gtk
       - test-gtk-ng
       - test-sentry-linux
       - test-macos
@@ -492,9 +491,6 @@ jobs:
       - name: test
         run: nix develop -c zig build -Dapp-runtime=none test
 
-      - name: Test GTK Build
-        run: nix develop -c zig build -Dapp-runtime=gtk -Demit-docs -Demit-webdata
-
       - name: Test GTK-NG Build
         run: nix develop -c zig build -Dapp-runtime=gtk-ng -Demit-docs -Demit-webdata
 
@@ -502,55 +498,6 @@ jobs:
       - name: Test System Build
         run: nix develop -c zig build --system ${ZIG_GLOBAL_CACHE_DIR}/p
 
-  test-gtk:
-    strategy:
-      fail-fast: false
-      matrix:
-        x11: ["true", "false"]
-        wayland: ["true", "false"]
-    name: GTK x11=${{ matrix.x11 }} wayland=${{ matrix.wayland }}
-    runs-on: namespace-profile-ghostty-sm
-    needs: test
-    env:
-      ZIG_LOCAL_CACHE_DIR: /zig/local-cache
-      ZIG_GLOBAL_CACHE_DIR: /zig/global-cache
-    steps:
-      - name: Checkout code
-        uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
-
-      - name: Setup Cache
-        uses: namespacelabs/nscloud-cache-action@a289cf5d2fcd6874376aa92f0ef7f99dc923592a # v1.2.17
-        with:
-          path: |
-            /nix
-            /zig
-
-      # Install Nix and use that to run our tests so our environment matches exactly.
-      - uses: cachix/install-nix-action@56a7bb7b56d9a92d4fd1bc05758de7eea4a370a8 # v31.6.0
-        with:
-          nix_path: nixpkgs=channel:nixos-unstable
-      - uses: cachix/cachix-action@0fc020193b5a1fa3ac4575aa3a7d3aa6a35435ad # v16
-        with:
-          name: ghostty
-          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
-
-      - name: Test
-        run: |
-          nix develop -c \
-            zig build \
-            -Dapp-runtime=gtk \
-            -Dgtk-x11=${{ matrix.x11 }} \
-            -Dgtk-wayland=${{ matrix.wayland }} \
-            test
-
-      - name: Build
-        run: |
-          nix develop -c \
-            zig build \
-            -Dapp-runtime=gtk \
-            -Dgtk-x11=${{ matrix.x11 }} \
-            -Dgtk-wayland=${{ matrix.wayland }}
-
   test-gtk-ng:
     strategy:
       fail-fast: false