net: ethernet: Remove accidental duplication in Kconfig file

Commit fb3dda82 ("net: airoha: Move airoha_eth driver in a dedicated
folder") accidentally added the line:

  source "drivers/net/ethernet/mellanox/Kconfig"

in drivers/net/ethernet/Kconfig, so that this line is duplicated in that
file.

Remove this accidental duplication.

Fixes: fb3dda82 ("net: airoha: Move airoha_eth driver in a dedicated folder")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20250306094753.63806-1-lukas.bulwahn@redhat.com


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