x86/apic: remove Summit support

IBM's Summit chipset was only used for 32-bit only Intel ccNUMA and
IA-64 machines, neither of which are supported by Xen anymore.

Signed-off-by: Matt Wilson <msw@amazon.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

docs/misc/xen-command-line.markdown

@@ -132,7 +132,7 @@ domain 0 command line
 Permit Xen to use superpages when performing memory management.
 
 ### apic
-> `= summit | bigsmp | default`
+> `= bigsmp | default`
 
 Override Xen's logic for choosing the APIC driver.  By default, if
 there are more than 8 CPUs, Xen will switch to `bigsmp` over