sfc: siena: rip out rss-context dead code

Siena hardware does not support custom RSS contexts, but when the
 driver was forked from sfc.ko, some of the plumbing for them was
 copied across from the common code.  Actually trying to use them
 would lead to EOPNOTSUPP as the relevant efx_nic_type methods were
 not populated.
Remove this dead code from the Siena driver.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240904181156.1993666-1-edward.cree@amd.com


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