override read-only when handling MPI messages

in `*shell*` buffers, the MPI message we are trying to extract may likely have
the special `read-only` text property, which prevents us from deleting it after
we handle it.

Overcome this by wrapping the deletion command in a let form that binds
`inhibit-read-only` to `t`, as suggested by the elisp manual's "Special
Properties" page

Author: dankessler

lisp/ess-tracebug.el

@@ -1257,7 +1257,8 @@ ends with an incomplete message."
                           (apply handler payload))
                       (error "No handler defined for MPI message '%s" head))
                   (goto-char mbeg0)
-                  (delete-region mbeg0 mend0)))
+                  (let ((inhibit-read-only t))
+                    (delete-region mbeg0 mend0))))
             (setq out :incomplete))))
       out)))