TextEditors: Remove useless duplicated (set)textCodec methods

They are duplicated from TextDocument::(set)codec. The default
implementation of the duplicated methods was just delegating to the text
document. The override of setTextCodec in QmlJsEditor was useless, since
it was only called from EditorConfiguration::configureEditor with the
ITextEditor::TextCodecFromProjectSetting flag anyhow, which made the
overridden method in QmlJsEditor be the same as the fallback.
Aside from that, the duplicated methods wouldn't have belonged to the
*editor* anyhow, but to the document.

Change-Id: Ib43c28210b6bf88726159d751a4905a1d062f80e
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>