ENH: Added removeAttribute function to MRML node comboboxes and proxy model

Attribute filtering in MRML node comboboxes work with only one attribute at a time. So filtering could be "disabled" for volumes by simply overwriting it with the labelmap attribute. As it does not exist any more, it was needed to add the corresponding removeAttribute function to qMRMLNodeComboBox and qMRMLSortFilterProxyModel (it was already implemented in qMRMLNodeFactory)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24425 3bd1e089-480b-0410-8dfb-8563597acbee