Clean exported headers of the Core plugin.

Change-Id: I26472d568844d5fee62323e01f5c5c12082d5450
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/coreplugin/variablechooser.h

@@ -38,8 +38,6 @@
 #include <utils/macroexpander.h>
 #include <functional>
 
-namespace Utils { class MacroExpander; }
-
 namespace Core {
 
 namespace Internal { class VariableChooserPrivate; }