intel/bd82x6x/Makefile.inc: Revert duplicate addition of `usb_debug.c`

intel/bd82x6x/Makefile.inc: Revert duplicate addition of `usb_debug.c`

Currently in Intel BD82x6x southbridge’s `Makefile.inc` the
file `usb_debug.c` is added twice to the build.

This was introduced in

    commit 4063ede3fb571110c3e65c321049cc2687cc54fa
    Author: Ronald G. Minnich <rminnich@google.com>
    Date:   Mon Feb 4 20:31:51 2013 -0800

        bd82x6x: Fix compiling with USB debug port support

        Reviewed-on: http://review.coreboot.org/2784

but was unneeded because it had been already added in
the following commit.

    commit 4141993536039e0d45caeacb745a89d388f0724b
    Author: Sven Schnelle <svens@stackframe.org>
    Date:   Sat Jul 28 08:52:44 2012 +0200

        bd82x6x: Fix CONFIG_USBDEBUG

        Reviewed-on: http://review.coreboot.org/1376

Therefore basically revert that hunk.

There is no policy on how to order these additions, so leave
it to a possible separate commit, unifying this.

Kyösti Mälkki suspects that these additions were meant for
the Intel Lynx Point [1].

[1] http://review.coreboot.org/#/c/3424/

Change-Id: Iaa8de6fcc0d6f3a0a92a28fcb603d7777aa8b24c
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/3425
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>