change some APIs for accepting utf8

source/charset.cpp

@@ -30,36 +30,6 @@
 
 namespace nana
 {
-	bool is_utf8(const char* str, unsigned len)
-	{
-		auto ustr = reinterpret_cast<const unsigned char*>(str);
-		auto end = ustr + len;
-
-		while (ustr < end)
-		{
-			const auto uv = *ustr;
-			if (uv < 0x80)
-			{
-				++ustr;
-				continue;
-			}
-
-			if (uv < 0xC0)
-				return false;
-
-			if ((uv < 0xE0) && (ustr + 1 < end))
-				ustr += 2;
-			else if (uv < 0xF0 && (ustr + 2 <= end))
-				ustr += 3;
-			else if (uv < 0x1F && (ustr + 3 <= end))
-				ustr += 4;
-			else
-				return false;
-		}
-
-		return true;
-	}
-
 	namespace detail
 	{
 		class locale_initializer