Remove the unused forward declaration for QDataStream from qglobal.h

The comment suggests that there should be function declarations that
need QDataStream, but there are none. It's probably a leftover from the
old code.

Change-Id: Iacefccc6b862d9a4ec111ebac8970b1ef01bcf39
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/corelib/global/qglobal.h

@@ -849,12 +849,6 @@ private:
 
 #endif // Q_OS_DARWIN
 
-/*
-   Data stream functions are provided by many classes (defined in qdatastream.h)
-*/
-
-class QDataStream;
-
 inline void qt_noop(void) {}
 
 /* These wrap try/catch so we can switch off exceptions later.