[master] pyexpat is not required

pyexpat is only required for unittest not for running
stats-httpd. (see ticket http://bind10.isc.org/ticket/2048)

doc/guide/bind10-guide.xml

@@ -132,9 +132,7 @@
         <command>b10-xfrout</command>, and <command>b10-zonemgr</command>
         components require the libpython3 library and the Python
         _sqlite3.so module (which is included with Python).
-        The <command>b10-stats-httpd</command> component uses the
-        Python pyexpat.so module.
-        The Python modules need to be built for the corresponding Python 3.
+        Python modules need to be built for the corresponding Python 3.
       </para>
 <!-- TODO: this will change ... -->