GDNative XR: remove redundant config.py

It's not an engine module. This is handled by GDNative's
`SCsub` instead, as done for other subdirectories already.

modules/gdnative/xr/config.py

@@ -1,6 +0,0 @@
-def can_build(env, platform):
-    return True
-
-
-def configure(env):
-    pass