Revert "fix a gc_wb of a jl_binding_t"

This reverts commit c343ca1.

carnaval already fixed this by making changes elsewhere, so it rebased
cleanly, but was incorrect

src/interpreter.c

@@ -402,7 +402,7 @@ static jl_value_t *eval(jl_value_t *e, jl_value_t **locals, size_t nl, size_t ng
  // temporarily assign so binding is available for field types
  check_can_assign_type(b);
  b->value = (jl_value_t*)dt;
- gc_wb_binding(((void**)b)-1, dt);
+ gc_wb_binding(b,dt);
 
  JL_TRY {
  // operations that can fail