Refactor FulfillmentError to track less data

Move the information about pointing at the call argument expression in
an unmet obligation span from the `FulfillmentError` to a new
`ObligationCauseCode`.

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -1998,7 +1998,6 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                                     &obligation,
                                     &traits::SelectionError::Unimplemented,
                                     false,
-                                    false,
                                 );
                             }
                         }