Remove null check inside not_null::get (#1067)

Guidelines issue 2006 removes the null check inside not_null::get, since the contained pointer is already guaranteed to be not-null upon construction.

Resolves #1051

include/gsl/pointers

@@ -117,7 +117,6 @@ public:
     not_null& operator=(const not_null& other) = default;
     constexpr details::value_or_reference_return_t<T> get() const
     {
-        Ensures(ptr_ != nullptr);
         return ptr_;
     }