DeleteChannels : Default channels plug to ""
DeleteChannels : Default channels plug to ""

This matches the behaviour of all our other Delete* nodes.

Breaking change :

- Changed default value for DeleteChannels channels plug

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