Documentation: efi: remove description of efi=old_map

The old EFI runtime region mapping logic that was kept around for some
time has finally been removed entirely, along with the SGI UV1 support
code that was its last remaining user. So remove any mention of the
efi=old_map command line parameter from the docs.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>