Remove obsolete instructions for building patched version of Xerces.
Remove obsolete instructions for building patched version of Xerces.

GPORCA works just fine with vanilla Xerces nowadays.

Reviewed-by: Chris Hajas <e8e414b713ceee89684f46616039fe9bed26bfcc@vmware.com>

greenplum-db/gpdb,jmcatamney/gpdb,lisakowen/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,jmcatamney/gpdb,adam8157/gpdb,lisakowen/gpdb,50wu/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,lisakowen/gpdb,adam8157/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,adam8157/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,jmcatamney/gpdb