Handle undo for deleting virtual field

Changes public API

    -QgsVectorLayerUndoCommandDeleteAttribute::QgsVectorLayerUndoCommandDeleteAttribute( QgsVectorLayerEditBuffer* buffer, int fieldIndex )
    +QgsVectorLayerUndoCommandDeleteAttribute::QgsVectorLayerUndoCommandDeleteAttribute( QgsVectorLayerEditBuffer* buffer, QgsExpressionFieldBuffer* expBuf, int fieldIndex )