Doc: Remove pre-Windows 7 from QCollator documentation

The code path that this refers to was removed already for Qt 5.11 in
commit 53fb2c48ef.

Change-Id: I4a7ae1b89b24c0ab7ceaa43f763c7ef422ca4900
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/corelib/tools/qcollator.cpp

@@ -222,11 +222,6 @@ Qt::CaseSensitivity QCollator::caseSensitivity() const
 
     By default this mode is off.
 
-    \note On Windows, this functionality makes use of the \l{ICU} library. If Qt was
-    compiled without ICU support, it falls back to code using native Windows API,
-    which only works from Windows 7 onwards. On older versions of Windows, it will not work
-    and a warning will be emitted at runtime.
-
     \sa numericMode()
  */
 void QCollator::setNumericMode(bool on)