MyGearmanWorkerImpl: fix RV_RETURN_VALUE_IGNORED_BAD_PRACTICE

Spotbugs complains about ExecutorService.submit() return value being
ignored. The returned Future is unneeded, just assign it to a variable
and forget about it.

src/main/java/hudson/plugins/gearman/MyGearmanWorkerImpl.java

@@ -634,7 +634,8 @@ private void submitFunction(GearmanFunction fun) {
             if (executorService == null) {
                 fun.call();
             } else {
-                executorService.submit(fun);
+                // The future is not used, we watch it via driveSessionIO()
+                Future task = executorService.submit(fun);
             }
 
             // We should have submitted either a WORK_EXCEPTION, COMPLETE,