Remove every trace of SERIAL_MAGIC

This means removing support for checking magic in amiserial.c
(SERIAL_PARANOIA_CHECK option), which was checking a magic field which
doesn't currently exist in the struct. That code hasn't built at least
since git.

Removing the definition from the header is safe anyway as that code was
from another driver and not including it.

Signed-off-by: Pascal Terjan <pterjan@google.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20191105192749.67533-1-pterjan@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/process/magic-number.rst

@@ -81,7 +81,6 @@ FF_MAGIC              0x4646           fc_info                  ``drivers/net/ip
 ISICOM_MAGIC          0x4d54           isi_port                 ``include/linux/isicom.h``
 PTY_MAGIC             0x5001                                    ``drivers/char/pty.c``
 PPP_MAGIC             0x5002           ppp                      ``include/linux/if_pppvar.h``
-SERIAL_MAGIC          0x5301           async_struct             ``include/linux/serial.h``
 SSTATE_MAGIC          0x5302           serial_state             ``include/linux/serial.h``
 SLIP_MAGIC            0x5302           slip                     ``drivers/net/slip.h``
 STRIP_MAGIC           0x5303           strip                    ``drivers/net/strip.c``