removes branch from motivation structure

img/motivation-structure.dot

 digraph {
     node [shape = "box", style = "filled, rounded"]
 
-    subgraph clustermaster {
-        label = "master"
-
-        node [color = limegreen]
-
     master4[label = "Dirk (today)\nupdates methods chapter", color = limegreen]
     master3[label = "Christian (yesterday)\nfixes typos", color = darkorchid1]
     master2[label = "Dirk (three days ago)\nadds methods chapter", color = limegreen]
@@ -13,4 +8,3 @@ digraph {
 
     master4 -> master3 -> master2 -> master1
 }
-}