QSlotObjectBase: remove misleading comment

QSlotObjectBase: remove misleading comment

These constructors might not benefit from being constexpr (as the objects are
only created on the heap), so don't suggest so. There's no disadvantage of them
begin constexpr, but their constexpr'ability depends on whether QAtomic has
a constexpr constructor, and the added complexity of finding that out isn't
worth it.

Change-Id: I089a29dcb98ba935c339dce09d71f283522a9afd
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>