net: spider_net: Remove powerpc Cell driver

This driver can no longer be built since support for IBM Cell Blades was
removed, in particular PPC_IBM_CELL_BLADE.

Remove the driver and the documentation.
Remove the MAINTAINERS entry, and add Ishizaki and Geoff to CREDITS.

Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20241218105523.416573-24-mpe@ellerman.id.au