[vcs-log] simplify code so that it would compile with all kotlin versions

platform/vcs-log/graph/test/com/intellij/vcs/log/graph/TestGraphBuilder.kt

@@ -258,7 +258,7 @@ class TestPermanentGraphInfo(
   }
 
   override fun getPermanentCommitsInfo() = commitInfo
-  override fun getLinearGraph() : PermanentLinearGraphImpl = object : PermanentLinearGraphImpl(), LinearGraph by graph {}
+  override fun getLinearGraph() = graph
   override fun getPermanentGraphLayout() = graphLayout
   override fun getBranchNodeIds() = branchNodes
 }