net: wan: Delete the DLCI / SDLA drivers

The DLCI driver (dlci.c) implements the Frame Relay protocol. However,
we already have another newer and better implementation of Frame Relay
provided by the HDLC_FR driver (hdlc_fr.c).

The DLCI driver's implementation of Frame Relay is used by only one
hardware driver in the kernel - the SDLA driver (sdla.c).

The SDLA driver provides Frame Relay support for the Sangoma S50x devices.
However, the vendor provides their own driver (along with their own
multi-WAN-protocol implementations including Frame Relay), called WANPIPE.
I believe most users of the hardware would use the vendor-provided WANPIPE
driver instead.

(The WANPIPE driver was even once in the kernel, but was deleted in
commit 8db60bcf30 ("[WAN]: Remove broken and unmaintained Sangoma
drivers.") because the vendor no longer updated the in-kernel WANPIPE
driver.)

Cc: Mike McLagan <mike.mclagan@linux.org>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
Link: https://lore.kernel.org/r/20201114150921.685594-1-xie.he.0141@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

CREDITS

@@ -2499,15 +2499,6 @@ W: http://www.rdrop.com/users/paulmck/
 D: RCU and variants
 D: rcutorture module
 
-N: Mike McLagan
-E: mike.mclagan@linux.org
-W: http://www.invlogic.com/~mmclagan
-D: DLCI/FRAD drivers for Sangoma SDLAs
-S: Innovative Logic Corp
-S: Post Office Box 1068
-S: Laurel, Maryland 20732
-S: USA
-
 N: Bradley McLean
 E: brad@bradpc.gaylord.com
 D: Device driver hacker