Cleaned out old references to removed buildhelpers and widgets modules. (#12383)

.gitignore

 /build/classes
 /build/test-output
 /build/integration-test-output
-/build/buildhelpers/com/vaadin/buildhelpers/*.class
 
 # /tests/
 /tests/junit*.properties
 server/target/
 client/target/
 client-compiler/target/
-buildhelpers/target/
 client-compiled/target/
 liferay/target/
 themes/target/
 uitest/target/
-widgets/target/
 
 .sass-cache
 phantomjsdriver.log

server/src/test/java/com/vaadin/tests/server/ClassesSerializableTest.java

 
 import org.junit.Test;
 
-import com.vaadin.ui.Component;
-
 public class ClassesSerializableTest {
 
     /**
             // class level filtering, also affecting nested classes and
             // interfaces
             "com\\.vaadin\\.server\\.LegacyCommunicationManager.*", //
-            "com\\.vaadin\\.buildhelpers.*", //
             "com\\.vaadin\\.util\\.EncodeUtil.*", //
             "com\\.vaadin\\.util\\.ReflectTools.*", //
             "com\\.vaadin\\.data\\.provider\\.InMemoryDataProviderHelpers",

test/servlet-containers/generic/ivysettings.xml

             resolver="vaadin-addons" />
         <module organisation="com.vaadin" name="vaadin-testbench-api"
             resolver="vaadin-maven" />
-        <module organisation="com.vaadin" name="vaadin-buildhelpers"
-            resolver="vaadin-maven" />
         <module organisation="com.vaadin" name="vaadin-root"
             resolver="vaadin-maven" />
         <module organisation="com.vaadin" name="vaadin-shared"
             resolver="vaadin-maven" />
         <module organisation="com.vaadin" name="vaadin-push"
             resolver="vaadin-maven" />
-        <module organisation="com.vaadin" name="vaadin-widgets"
-            resolver="vaadin-maven" />
         <module organisation="com.vaadin" name="vaadin-bom"
             resolver="vaadin-maven" />
         <module organisation="com.vaadin" name="vaadin-liferay"