mlxsw: Remove Mellanox SwitchX-2 ASIC support

Initial support for the Mellanox SwitchX-2 ASIC was added in July 2015.
Since then all development efforts shifted towards the Mellanox Spectrum
ASICs and development of this driver stopped beside trivial fixes and
refactoring. Therefore, the driver does not support any switch offloads
and simply traps all traffic to the CPU, rendering it irrelevant for
deployment.

In addition, support for this ASIC was dropped by Mellanox a few years
ago.

Given the driver is not used by any users and that there is no
intention of investing in its development, remove it from the kernel.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>