Flatpak: Upgrade kde runtime to 6.8

this version bundles SDL2-2.30.6, the temporary measure of building the
vendored version from exports is no longer necessary.

Flatpak/SDL2/SDL2.json

@@ -1,22 +0,0 @@
-{
-  "name": "SDL2",
-  "buildsystem": "autotools",
-  "config-opts": ["--disable-static"],
-  "sources": [
-    {
-      "type": "dir",
-      "path": "../../Externals/SDL/SDL"
-    }
-  ],
-  "cleanup": [ "/bin/sdl2-config",
-               "/include",
-               "/lib/libSDL2.la",
-               "/lib/libSDL2main.a",
-               "/lib/libSDL2main.la",
-               "/lib/libSDL2_test.a",
-               "/lib/libSDL2_test.la",
-               "/lib/cmake",
-               "/share/aclocal",
-               "/lib/pkgconfig"]
-}
-

Flatpak/org.DolphinEmu.dolphin-emu.yml

@@ -1,6 +1,6 @@
 app-id: org.DolphinEmu.dolphin-emu
 runtime: org.kde.Platform
-runtime-version: '6.7'
+runtime-version: '6.8'
 sdk: org.kde.Sdk
 command: dolphin-emu-wrapper
 rename-desktop-file: dolphin-emu.desktop
@@ -47,9 +47,6 @@ modules:
         url: https://github.com/Unrud/xdg-screensaver-shim/archive/0.0.2.tar.gz
         sha256: 0ed2a69fe6ee6cbffd2fe16f85116db737f17fb1e79bfb812d893cf15c728399
 
-  # build the vendored SDL2 from Externals until the runtime gets 2.30.6
-  - SDL2/SDL2.json
-
   - name: dolphin-emu
     buildsystem: cmake-ninja
     config-opts: