x86/apic/uv: Remove code for unused distributed GRU mode

Distributed GRU mode appeared in only one generation of UV hardware,
and no version of the BIOS has shipped with this feature enabled, and
we have no plans to ever change that.  The gru.s3.mode check has
always been and will continue to be false.  So remove this dead code.

Signed-off-by: Steve Wahl <steve.wahl@hpe.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Dimitri Sivanich <sivanich@hpe.com>
Link: https://lkml.kernel.org/r/20200513221123.GJ3240@raspberrypi