net: wan: remove support for COSA and SRP synchronous serial boards

Looks like all the changes to this driver had been automated
churn since git era begun. The driver is using virt_to_bus()
so it should be updated to a proper DMA API or removed. Given
the latest "news" entry on the website is from 1999 I'm opting
for the latter.

I'm marking the allocated char device major number as [REMOVED],
I reckon we can't reuse it in case some SW out there assumes its
COSA?

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>