make butil::BasicStringPiece<T> support string split functions-family

src/butil/strings/string_util.cc

@@ -373,10 +373,6 @@ bool IsStringUTF8(const StringPiece& str) {
   return true;
 }
 
-bool IsStringUTF8(const std::string& str) {
-  return IsStringUTF8(StringPiece(str));
-}
-
 }  // namespace butil
 
 template<typename Iter>

src/butil/strings/string_util.h

@@ -254,7 +254,6 @@ BUTIL_EXPORT bool ContainsOnlyChars(const StringPiece16& input,
 // to have the maximum 'discriminating' power from other encodings. If
 // there's a use case for just checking the structural validity, we have to
 // add a new function for that.
-BUTIL_EXPORT bool IsStringUTF8(const std::string& str);
 BUTIL_EXPORT bool IsStringUTF8(const StringPiece& str);
 BUTIL_EXPORT bool IsStringASCII(const StringPiece& str);
 BUTIL_EXPORT bool IsStringASCII(const string16& str);