sis-agp: Remove SIS 760, handled by amd64-agp

SIS 760 is listed in the device tables for both amd64-agp and sis-agp.
amd64-agp is apparently preferable since it has workarounds for some
BIOS misconfigurations that sis-agp doesn't handle.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>