xen/x86: p2m: Don't initialize slot 0 of the P2M

xen/x86: p2m: Don't initialize slot 0 of the P2M

It is not entirely clear why the slot 0 of each p2m should be populated
with empty page-tables. The commit introducing it 759af8e3800 "[HVM]
Fix 64-bit HVM domain creation." does not contain meaningful
explanation except that it was necessary for shadow.

As we don't seem to have a good explanation why this is there, drop the
code completely.

This was tested by successfully booting a HVM with shadow enabled.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>