Don't use -html4 for javadoc output for Ant classes

javadoc on Ant classes now conform to the default output format of javadoc tool
(which is HTML5 in recent versions of that tool), so we no longer have to
override the output format to -html4.

build.xml

@@ -1503,14 +1503,6 @@
         <patternset refid="useful.tests"/>
       </srcfiles>
     </uptodate>
-    <!-- Javadoc on classes in Ant project, don't (yet) comply to HTML5
-    semantics. javadoc tool in JDK 9+ allows specifying the HTML version
-    to use for the output generation. We intentionally set it to -html4
-    to prevent javadoc tool failures (in JDK 11, which is strict) when
-    running against our code -->
-    <condition property="javadoc.output.htmlversion" value="-html4" else="">
-      <isset property="jdk9+"/>
-    </condition>
   </target>
 
   <target name="javadocs" depends="check-javadoc"
@@ -1543,7 +1535,6 @@
       <group title="Optional Types" packages="org.apache.tools.ant.types.optional*"/>
       <group title="Ant Utilities" packages="org.apache.tools.ant.util*"/>
       <classpath refid="tests-classpath"/>
-      <arg line="${javadoc.output.htmlversion}"/>
     </javadoc>
   </target>