[sledge] Build: Disable warning 30

Summary:
It is not needed or very meaningful since the addition of type-based
disambiguation.

Reviewed By: jvillard

Differential Revision: D22401035

fbshipit-source-id: 31996f946

sledge/dune

@@ -6,7 +6,7 @@
 (env
  (debug
   (flags
-   (-w +a-4-9-18-40-42-44-48@50-66 -strict-formats -strict-sequence
+   (-w +a-4-9-18-30-40-42-44-48@50-66 -strict-formats -strict-sequence
      -error-style short -short-paths -bin-annot -keep-locs -keep-docs -opaque))
   (env-vars
    (PPX_TRACE_ENABLED 1))