QUrl: remove unused qt_from_latin1() forward declaration

It's not currently used in qurl.cpp.

Change-Id: Ic654e28fc64d90897be3a38d477e38de0414a02a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qurl.cpp

@@ -405,9 +405,6 @@ QT_BEGIN_NAMESPACE
 
 using namespace Qt::StringLiterals;
 
-// in qstring.cpp:
-void qt_from_latin1(char16_t *dst, const char *str, size_t size) noexcept;
-
 inline static bool isHex(char c)
 {
     c |= 0x20;