net: ethtool: remove the compat code for _rxfh_context ops

All drivers are now converted to dedicated _rxfh_context ops.
Remove the use of >set_rxfh() to manage additional contexts.

Reviewed-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Edward Cree <ecree.xilinx@gmail.com>
Link: https://patch.msgid.link/20250707184115.2285277-5-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>