x86/PV: drop dead paging_update_paging_modes() call during Dom0 construction

x86/PV: drop dead paging_update_paging_modes() call during Dom0 construction

The function won't ever be invoked, as paging_mode_enabled() always
returns false here due to the immediately preceding clearing of
d->arch.paging.mode. While compilers recognize this and eliminate the
call, make this explicit in the source (which likely 9a28170f2da2 ["pvh
dom0: construct_dom0 changes"] should have done right away, albeit even
before that the call looks to have been pointless - shadow mode enabling
has occurred later virtually forever).

While there also update an adjacent partly stale comment.

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