flatpak: Remove meson build

GNOME SDK contains Meson since version 3.38.

See e8ea702691

build/flatpak/org.gimp.GIMP-nightly.json

@@ -21,20 +21,6 @@
                 "/share/aclocal", "/man", "/share/man", "/share/gtk-doc",
                 "/share/vala", "*.la", "*.a" ],
     "modules": [
-        {
-            "name": "meson",
-            "buildsystem": "simple",
-            "config-opts": [ "-Dintrospection=false" ],
-            "cleanup": ["*"],
-            "sources": [
-                {
-                    "type": "archive",
-                    "url": "https://github.com/mesonbuild/meson/releases/download/0.54.0/meson-0.54.0.tar.gz",
-                    "sha256": "dde5726d778112acbd4a67bb3633ab2ee75d33d1e879a6283a7b4a44c3363c27"
-                }
-            ],
-            "build-commands": [ "python3 setup.py install --prefix=/app" ]
-        },
         {
             "name": "gexiv2",
             "modules": [