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/git/gitplugin.cpp

@@ -1053,7 +1053,7 @@ void GitPlugin::submitCurrentLog()
 {
     // Close the submit editor
     m_submitActionTriggered = true;
-    Core::ICore::editorManager()->closeEditor();
+    Core::EditorManager::closeEditor();
 }
 
 bool GitPlugin::submitEditorAboutToClose()