removes libusb module dependency from flatpak manifest

flatpak/org.sdrangel.SDRangel.json

@@ -15,15 +15,6 @@
     "--socket=wayland"
   ],
   "modules": [
-    {
-      "name": "libusb",
-      "sources": [
-        {
-          "type": "git",
-          "url": "https://github.com/libusb/libusb.git"
-        }
-      ]
-    },
     {
       "name": "cm256cc",
       "buildsystem": "cmake-ninja",