AnimationEditor : Refactor away `__visiblePlugs` member data
AnimationEditor : Refactor away `__visiblePlugs` member data

It was never used outside of `__expansionChanged()`, and we weren't using it to carry state from one invocation to the next.

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