Simplify the 5.3.0 support

Author: samoht

lib/top/compat_top.ml

@@ -97,9 +97,7 @@ let extension_constructor
   ; ext_loc
   ; ext_attributes
 #if OCAML_VERSION >= (5, 3, 0)
-  ; ext_uid = Uid.mk
-      ~current_unit:
-        (Some (Unit_info.make ~source_file:"mdx.ml" Impl "mdx"))
+  ; ext_uid = Uid.mk ~current_unit:None
 #elif OCAML_VERSION >= (4, 11, 0)
   ; ext_uid = Uid.mk ~current_unit:"mdx"
 #endif