Revert "Use widget set specified by init parameter (#20276)"

Breaks all use of AppWidgetset together with
@VaadinServletConfiguration

This reverts commit 516c428ca1.

Change-Id: I9e9bd792479ee1f4dde08c5d2a4d87e06b77f519

server/src/main/java/com/vaadin/server/UIProvider.java

             return new WidgetsetInfoImpl(uiWidgetset.value());
         }
 
-        // Second case: We might have an init parameter, use that
-        String initParameterWidgetSet = event.getService()
-                .getDeploymentConfiguration().getWidgetset(null);
-        if (initParameterWidgetSet != null) {
-            return new WidgetsetInfoImpl(initParameterWidgetSet);
-        }
-
         // Find the class AppWidgetset in the default package if one exists
         WidgetsetInfo info = getWidgetsetClassInfo();
 
-        // Third case: we have a generated class called APP_WIDGETSET_NAME
+        // Second case: we have a generated class called APP_WIDGETSET_NAME
         if (info != null) {
             return info;
         }
 
-        // Fourth case: we have an AppWidgetset.gwt.xml file
+        // third case: we have an AppWidgetset.gwt.xml file
         else {
             InputStream resource = event.getUIClass()
                     .getResourceAsStream("/" + APP_WIDGETSET_NAME + ".gwt.xml");