LUCENE-9090: remove ant runtime pollution from tests classpath

previously, entire classpath of ant (ant itself, plugins, ivy, etc) were
polluting the unit tests classpath. it leads to non-reproducible build
issues because tests classpath is different depending on things outside
of source code control.

for example, solr tests launching hadoop, hadoop launching jetty, jetty
scanning classpath -> boom

lucene/common-build.xml

@@ -2549,7 +2549,6 @@ The following arguments can be provided to ant to alter its behaviour and target
         <path id="pitest.classpath">
             <path refid="junit.classpath"/>
             <path refid="pitest.framework.classpath"/>
-            <pathelement path="${java.class.path}"/>
         </path>
 
         <junit4:pickseed property="pitest.seed" />

lucene/core/build.xml

@@ -41,7 +41,6 @@
 
   <path id="junit.classpath">
     <path refid="test.classpath"/>
-    <pathelement path="${java.class.path}"/>
   </path>
 
   <target name="test-core" depends="common.test"/>

lucene/module-build.xml

@@ -47,7 +47,6 @@
   <path id="junit.classpath">
     <pathelement location="${build.dir}/classes/test"/>
     <path refid="test.classpath"/>
-    <pathelement path="${java.class.path}"/>
   </path>
 
   <target name="init" depends="common.init,compile-lucene-core"/>

lucene/test-framework/build.xml

@@ -39,7 +39,6 @@
 
   <path id="junit.classpath">
     <path refid="test.classpath"/>
-    <pathelement path="${java.class.path}"/>
   </path>
 
   <!--