Death to Multibuffer extension

The rationale behind is because no sane application will use this when we have
modern APIs such DRI2. Besides, as a fact, xfree86 server has already
deprecated this extension in 1998:

    http://www.xfree86.org/3.3.6/isc7.html

Signed-off-by: Tiago Vignatti <tiago.vignatti@nokia.com>
Reviewed-by: Keith Packard <keithp@keithp.com>

os/utils.c

@@ -148,9 +148,6 @@ Bool noScreenSaverExtension = FALSE;
 #ifdef MITSHM
 Bool noMITShmExtension = FALSE;
 #endif
-#ifdef MULTIBUFFER
-Bool noMultibufferExtension = FALSE;
-#endif
 #ifdef RANDR
 Bool noRRExtension = FALSE;
 #endif