Remove defunct compile option CONFIG_XEN_SHADOW_MODE.

Remove defunct compile option CONFIG_XEN_SHADOW_MODE.
It has been superceded by run-time flag
XENFEAT_auto_translated_physmap.

This patch changes behaviour slightly:
 1. set_pmd/set_pgd/set_pud always use mmu_ops, even when
    XENFEAT_writable_page_tables is available. Unclear whether
    we want to add a test to avoid this.
 2. xen_l1_entry_update always uses mmu_op. Again, could gate
    this on XENFEAT_writable_page_tables.
 3. We always pin/unpin pagetables. Could gate this on a
    new XENFEAT flag.

Signed-off-by: Keir Fraser <keir@xensource.com>