Don't register mimetype for diff editor

Task-number: QTCREATORBUG-11624

Change-Id: I9eeeeeb862e335c1a4d5cbd80d0908297ad6d1db
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/diffeditor/diffeditordocument.cpp

@@ -35,9 +35,8 @@
 
 namespace DiffEditor {
 
-DiffEditorDocument::DiffEditorDocument(const QString &mimeType) :
+DiffEditorDocument::DiffEditorDocument() :
     Core::IDocument(),
-    m_mimeType(mimeType),
     m_diffEditorController(new DiffEditorController(this))
 {
     setDisplayName(QCoreApplication::translate("DiffEditor", Constants::DIFF_EDITOR_DISPLAY_NAME));
@@ -69,7 +68,7 @@ bool DiffEditorDocument::save(QString *errorString, const QString &fileName, boo
 
 QString DiffEditorDocument::mimeType() const
 {
-    return m_mimeType;
+    return QString();
 }
 
 Core::IDocument::ReloadBehavior DiffEditorDocument::reloadBehavior(ChangeTrigger state, ChangeType type) const