MAINTAINERS: update enic and usnic maintainers

Remove Christian Benvenuti from the enic/usnic maintainer lists because he
is no longer working on the drivers and the email address is no longer valid.
Keep Satish Kharat as the enic maintainer and the usnic co-maintainer.

Signed-off-by: Satish Kharat <satishkh@cisco.com>
Acked-by: Leon Romanovsky <leon@kernel.org>
Link: https://patch.msgid.link/20260219-enic-maintainers-update-v2-v1-1-c58aa11c2ea8@cisco.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>