Make the exception-destruction rationale self-contained

Removing the speculative destruct_exn.md design note left this docstring
pointing at a deleted file. The note's surviving conclusion - one cannot
soundly ask whether an arbitrary value is an exception, and try/catch
never needs to - now lives in the comment itself.

Signed-Off-By: Cristiano Calcagno <ccrisccris@gmail.com>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PCtQiaDijUqA2fujQXvKUw

Author: cristianoc

packages/@rescript/runtime/Primitive_exceptions.res

@@ -40,7 +40,11 @@ type js_error = {cause: exn}
      | None -> (* assert it is not an exception *)
    ]}
 
-   This is not a problem in `try .. with` since the logic above is not expressible, see more design in [destruct_exn.md]
+   This is not a problem in `try .. with`: a handler only asks whether the
+   caught value matches an exception branch, never whether an arbitrary value
+   is an exception - the question a general exception-destruction operator
+   would force, and which cannot be answered soundly while open variants
+   share the exception representation.
 */
 let isExtension = (type a, e: a): bool =>
   if Primitive_js_extern.testAny(e) {