Remove checkstyle "FinalClass" (#10145)

checkstyle/vaadin-checkstyle.xml

@@ -241,7 +241,6 @@
         <module name="DesignForExtension">
             <property name="severity" value="ignore" />
         </module>
-        <module name="FinalClass" />
         <module name="HideUtilityClassConstructor" />
         <module name="InterfaceIsType" />
         <module name="VisibilityModifier">