build: Properly clean up grass

This is the right way, removing all files generated by the module.

build-aux/org.gnome.Fractal.Devel.json

@@ -48,9 +48,6 @@
     "cleanup-commands": [
         "mkdir -p ${FLATPAK_DEST}/lib/ffmpeg"
     ],
-    "cleanup": [
-        "/bin/grass"
-    ],
     "modules": [
         {
             "name": "grass",
@@ -65,6 +62,7 @@
                 "mkdir -p /app/bin",
                 "install -D ./target/release/grass /app/bin/"
             ],
+            "cleanup": ["*"],
             "sources": [
                 {
                     "type": "git",