docs/ia64: Drop obsolete Xen documentation

While the xensource.com URLs referenced still exist, neither the Xen or Linux
2.6.18 fork have been touched since 2009, 11 years ago.  Other URLs are dead.

IA64 support was removed in Xen 4.2, in 2012.  Relegate this piece of
documentation to source history.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Link: https://lore.kernel.org/r/20200827175405.24344-1-andrew.cooper3@citrix.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>