Removed span from-nullptr_t ctor.

include/gsl/string_span

@@ -203,9 +203,6 @@ public:
     // move assign
     constexpr basic_string_span& operator=(basic_string_span&& other) GSL_NOEXCEPT = default;
 
-    // from nullptr
-    constexpr basic_string_span(std::nullptr_t ptr) GSL_NOEXCEPT : span_(ptr) {}
-
     constexpr basic_string_span(pointer ptr, index_type length) : span_(ptr, length) {}
     constexpr basic_string_span(pointer firstElem, pointer lastElem) : span_(firstElem, lastElem) {}