[hotfix][scheduler] Remove the unused method and add the @nullable inside the WaitingForResources

Author: 1996fanrui

flink-runtime/src/main/java/org/apache/flink/runtime/scheduler/adaptive/WaitingForResources.java

@@ -237,7 +237,7 @@ public Factory(
                 Logger log,
                 Duration initialResourceAllocationTimeout,
                 Duration resourceStabilizationTimeout,
-                ExecutionGraph previousExecutionGraph) {
+                @Nullable ExecutionGraph previousExecutionGraph) {
             this.context = context;
             this.log = log;
             this.initialResourceAllocationTimeout = initialResourceAllocationTimeout;
@@ -259,9 +259,4 @@ public WaitingForResources getState() {
                     previousExecutionGraph);
         }
     }
-
-    @Nullable
-    public ExecutionGraph getPreviousExecutionGraph() {
-        return previousExecutionGraph;
-    }
 }