Doc fix for removed var gdb-use-separate-io-buffer.

* building.texi (GDB User Interface Layout): Remove reference to
removed variable gdb-use-separate-io-buffer.

Fixes: debbugs:12454

doc/emacs/ChangeLog

+2012-09-17  Chong Yidong  <cyd@gnu.org>
+
+	* building.texi (GDB User Interface Layout): Remove reference to
+	removed variable gdb-use-separate-io-buffer (Bug#12454).
+
 2012-09-08  Jambunathan K  <kjambunathan@gmail.com>
 
 	* regs.texi (Text Registers): `C-x r +' can now be used instead of

doc/emacs/building.texi

@@ -888,10 +888,6 @@ displays the following frame layout:
 @end group
 @end smallexample
 
-  However, if @code{gdb-use-separate-io-buffer} is @code{nil}, the I/O
-buffer does not appear and the primary source buffer occupies the full
-width of the frame.
-
 @findex gdb-restore-windows
 @findex gdb-many-windows
   If you ever change the window layout, you can restore the ``many