[Attributes] Remove nonnull from UB-implying attributes

From LangRef:

> if the parameter or return pointer is null, poison value is
> returned or passed instead. The nonnull attribute should be
> combined with the noundef attribute to ensure a pointer is not
> null or otherwise the behavior is undefined.

Dropping noundef is sufficient to prevent UB. Including nonnull
in this method just muddies the semantics.