x86: nuke PV superpage option and code

x86: nuke PV superpage option and code

Delete the user visible option and code for PV superpage support. The
mm code is modified as if the option is set to false (the default
value).

Return the address space occupied by spage_info back to the reserved
address space.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>