x86/shadow: Drop dubious lastpage diagnostic

x86/shadow: Drop dubious lastpage diagnostic

This is a global variable (actually 3, one per GUEST_PAGING_LEVEL), operated
on using atomics only (with no regard to what else shares the same cacheline),
which emits a diagnostic (in debug builds only) without changing any program
behaviour.

It is presumably left-over debugging, as it interlinks the behaviour of all
vCPUs in chronological order.  Based on the read-only p2m types, this
diagnostic can be tripped by entirely legitimate guest behaviour.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>