Revert "DiffEditorController: Add an option to set a syntax highlighter"

This reverts commit 364beabcf1.

Reason for revert: Not needed anymore after bf823eb581

Change-Id: I8f1d4db7d8b3a216e808ec431f002c1199fb919c
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: André Hartmann <aha_1980@gmx.de>

src/plugins/diffeditor/diffeditordocument.cpp

@@ -179,13 +179,6 @@ void DiffEditorDocument::setDescription(const QString &description)
     emit descriptionChanged();
 }
 
-void DiffEditorDocument::setDescriptionSyntaxHighlighterCreator(
-    const std::function<TextEditor::SyntaxHighlighter *()> &creator)
-{
-    m_descriptionHighlighter = creator;
-    emit descriptionHighlighterChanged();
-}
-
 QString DiffEditorDocument::description() const
 {
     return m_description;