flatpak: Do not use builddir

This currently leads to Builder rebuilding for every branch.
With builddir=false we get incremental builds across branches.

build-aux/flatpak/org.gnome.Loupe.Devel.json

@@ -96,7 +96,6 @@
         },
         {
             "name" : "loupe",
-            "builddir" : true,
             "buildsystem" : "meson",
             "config-opts": [
                 "-Dprofile=development"