kernel: remove mvsw61xx swconfig driver

[dengqf6 at mail2.sysu.edu.cn]

Hi,
 
> I assume this will land in kernel 5.11.
> 
> At a minimum, it needs to be backported.

I already backported it, see commit 24410595e91c.

But there is no uci configuration available yet, you have to use `tc' in command line.
Refer to https://github.com/Mellanox/mlxsw/wiki/Port-Mirroring#user-content-basic-configuration for the configuration.