DiffEditorManager simplify

by using EditorManager::openEditorWithContents

Change-Id: I2850648bff0f5b8f01254b9c7ce9539c1262f576
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/diffeditor/diffeditor.pro

@@ -7,7 +7,6 @@ HEADERS += diffeditor_global.h \
         diffeditorcontroller.h \
         diffeditordocument.h \
         diffeditorfactory.h \
-        diffeditormanager.h \
         diffeditorplugin.h \
         differ.h \
         diffutils.h \
@@ -20,7 +19,6 @@ SOURCES += diffeditor.cpp \
         diffeditorcontroller.cpp \
         diffeditordocument.cpp \
         diffeditorfactory.cpp \
-        diffeditormanager.cpp \
         diffeditorplugin.cpp \
         differ.cpp \
         diffutils.cpp \