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 8db60bcf

 ("[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 because the vendor no longer updated the in-kernel WANPIPE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>