Core: Remove unneeded ifdefs.

Replace compile-time checks for host OS by run-time checks.

Change-Id: I9f237389171586786c2609f81314bcb1bc17b01e
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/coreplugin/mainwindow.h

@@ -120,9 +120,7 @@ public:
 
     void setOverrideColor(const QColor &color);
 
-#ifdef Q_OS_MAC
     void setIsFullScreen(bool fullScreen);
-#endif
 signals:
     void windowActivated();