docbook: don't use serial_core.h in device-drivers book

Fix new kernel-doc warning.  This file no longer contains
kernel-doc comments.

Warning(include/linux/serial_core.h): no structured comments found
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/DocBook/device-drivers.tmpl

@@ -216,7 +216,6 @@ X!Isound/sound_firmware.c
 
   <chapter id="uart16x50">
      <title>16x50 UART Driver</title>
-!Iinclude/linux/serial_core.h
 !Edrivers/tty/serial/serial_core.c
 !Edrivers/tty/serial/8250.c
   </chapter>