Clean up some code in PlatformSpecRunner

- In 'runFeature', the 'if (currentFeature.isSkipped())' code block is
  unnecessary, because it will never be executed, as 'runFeature' also
  will not be executed for skipped features.

- In 'runSpec', add 'if (spec.isExcluded()) return;' analogous to
  the existing 'if (currentFeature.isExcluded()) return;' in
  'runFeature', logically aligning the code structures in both methods.

Relates to spockframework#1662.

spock-core/src/main/java/org/spockframework/runtime/PlatformSpecRunner.java

@@ -51,6 +51,8 @@ public void runSpec(SpockExecutionContext context, Runnable specRunner) {
     if (context.getErrorInfoCollector().hasErrors()) return;
 
     SpecInfo spec = context.getSpec();
+    if (spec.isExcluded()) return;
+
     supervisor.beforeSpec(spec);
     invoke(context, this, createMethodInfoForDoRunSpec(context, specRunner));
     supervisor.afterSpec(spec);
@@ -183,11 +185,6 @@ public void runFeature(SpockExecutionContext context, Runnable feature) {
     FeatureInfo currentFeature = context.getCurrentFeature();
     if (currentFeature.isExcluded()) return;
 
-    if (currentFeature.isSkipped()) {
-      supervisor.featureSkipped(currentFeature); // todo notify in node isSkipped
-      return;
-    }
-
     supervisor.beforeFeature(currentFeature);
     invoke(context, this, createMethodInfoForDoRunFeature(context, feature));
     supervisor.afterFeature(currentFeature);