net: x25_asy: Delete the x25_asy driver

This driver transports LAPB (X.25 link layer) frames over TTY links.

I can safely say that this driver has no actual user because it was
not working at all until:
commit 8fdcabea ("drivers/net/wan/x25_asy: Fix to make it work")

The code in its current state still has problems:

1.
The uses of "struct x25_asy" in x25_asy_unesc (when receiving) and in
x25_asy_write_wakeup (when sending) are not protected by locks against
x25_asy_change_mtu's changing of the transmitting/receiving buffers.
Also, all "netif_running" checks in this driver are not protected by
locks against the ndo_stop function.

2.
The driver stops all TTY read/write when the netif is down.
I think this is not right because this may cause the last outgoing frame
before the netif goes down to be incompletely transmitted, and the first
incoming frame after the netif goes up to be incompletely received.

And there may also be other problems.

I was planning to fix these problems but after recent discussions about
deleting other old networking code, I think we may just delete this
driver, too.

Signed-off-by: Xie He <xie.he.0141@gmail.com>
Acked-by: Martin Schiller <ms@dev.tdt.de>
Link: https://lore.kernel.org/r/20201105073434.429307-1-xie.he.0141@gmail.com


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