LabelPlugValueWidget : Remove custom label when renaming plug.
LabelPlugValueWidget : Remove custom label when renaming plug.

Fixes #1635.

chippey/gaffer,hradec/gaffer,ivanimanishi/gaffer,chippey/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,hradec/gaffer,boberfly/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,lucienfostier/gaffer,chippey/gaffer,ivanimanishi/gaffer,hradec/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,boberfly/gaffer,boberfly/gaffer,ivanimanishi/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,appleseedhq/gaffer,boberfly/gaffer,andrewkaufman/gaffer,hradec/gaffer,ivanimanishi/gaffer,chippey/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,hradec/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,lucienfostier/gaffer