all: delete the ui.Op type

It's no longer used.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/draw/draw.go

@@ -69,8 +69,6 @@ func (i *OpImage) Decode(d []byte, refs []interface{}) {
 	}
 }
 
-func (OpImage) ImplementsOp() {}
-
 // RectPath constructs a path corresponding to
 // a pixel aligned rectangular area.
 func RectPath(r image.Rectangle) *Path {

ui/draw/path.go

@@ -280,5 +280,3 @@ func (p *PathBuilder) Path() *Path {
 	}
 	return data
 }
-
-func (p OpClip) ImplementsOp() {}