Hide servlet features, should be selected via --servlet instead

the servlet container should be selected
via --servlet
jetty      none       tomcat     undertow

Selecting via features does not properly override ServletImpl

Author: jamesfredley

grails-forge/grails-forge-core/src/main/java/org/grails/forge/feature/spring/SpringBootEmbeddedServlet.java

@@ -33,4 +33,9 @@ public Class<?> getFeatureClass() {
     public String getCategory() {
         return Category.SERVER;
     }
+
+    @Override
+    public boolean isVisible() {
+        return false;
+    }
 }