Remove the ill-considered StringRef::numberOfCharacters

It never counted characters, only codepoints. If you think these
are interchangeable, you should not touch non-ascii text.

include/internal/catch_stringref.h

@@ -96,7 +96,6 @@ namespace Catch {
             return m_size;
         }
 
-        auto numberOfCharacters() const noexcept -> size_type;
         auto c_str() const -> char const*;
 
     public: // substrings and searches