x86/domain: Drop the only-written smap_check_policy infrastructure

x86/domain: Drop the only-written smap_check_policy infrastructure

c/s 4c5d78a10d "x86/pagewalk: Re-implement the pagetable walker" dropped the
consumer of smap_policy.  Looking at c/s 31ae587e6f which introduced the
smap_check logic, it exists only to work around a bug in guest_walk_tables()
was resolved by the aformentioned commit.

Remove the unused variables and associated infrastructure.

Reported-by: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>