graph.go: remove unused Reticle code

graph.go

@@ -29,7 +29,6 @@ type Level []*NodeRef
 type Reticle struct {
 	levels      []Level
 	left, right int
-	charWidth   int
 	colsWidth   []int
 }
 
@@ -59,10 +58,6 @@ func (r *Reticle) computeCharWidth() {
 			r.colsWidth[c] = max(r.colsWidth[c], len(ref.label)+2) // +2 to make room for brakets
 		}
 	}
-	r.charWidth = 0
-	for _, w := range r.colsWidth {
-		r.charWidth += w
-	}
 }
 
 func (r *Reticle) build(node *term, depth, pos int) {