EditorManager: Use interface directly instead of going through ICore

In the majority of cases we were doing that anyways, having two
ways is just needlessly confusing.

Change-Id: Ied362a702c23beee528368d74df1f2aabe5807f8
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/coreplugin/icore.cpp

@@ -422,11 +422,6 @@ MessageManager *ICore::messageManager()
     return m_mainwindow->messageManager();
 }
 
-EditorManager *ICore::editorManager()
-{
-    return m_mainwindow->editorManager();
-}
-
 ProgressManager *ICore::progressManager()
 {
     return m_mainwindow->progressManager();