flatpak: Stop bundling copy of libsecret

We started bundling libsecret in commit a085f90831 to workaround an
issue in the 0.20 version where looking up a password could indefinitely
hang.

Since version 0.21.4, this has been resolved, and this version is also
available in the GNOME runtime, so let's stop building it ourselves.

org.gnome.Geary.json

@@ -205,20 +205,6 @@
                 "--disable-oss"
             ]
         },
-        {
-            "name": "libsecret",
-            "buildsystem": "meson",
-            "config-opts": [
-                "-Dgtk_doc=false"
-            ],
-            "sources": [
-                {
-                    "type": "archive",
-                    "url": "https://download.gnome.org/sources/libsecret/0.21/libsecret-0.21.4.tar.xz",
-                    "sha256": "163d08d783be6d4ab9a979ceb5a4fecbc1d9660d3c34168c581301cd53912b20"
-                }
-            ]
-        },
         {
             "name": "gsound",
             "buildsystem": "meson",