gallium/x11: remove empty GLX_SGIX_swap_group stubs

The extension was never implemented. Quick search suggests:
 - no actual users (on my Arch setup)
 - the Nvidia driver does not implement the extension

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Ian Romanick <ian.d.romanick@intel.com>
Acked-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>

src/gallium/state_trackers/glx/xlib/glx_api.c

@@ -2647,18 +2647,6 @@ glXAssociateDMPbufferSGIX(Display *dpy, GLXPbufferSGIX pbuffer,
 #endif
 
 
-/*** GLX_SGIX_swap_group ***/
-
-PUBLIC void
-glXJoinSwapGroupSGIX(Display *dpy, GLXDrawable drawable, GLXDrawable member)
-{
-   (void) dpy;
-   (void) drawable;
-   (void) member;
-}
-
-
-
 /*** GLX_SUN_get_transparent_index ***/
 
 PUBLIC Status

src/gallium/state_trackers/glx/xlib/glx_getproc.c

@@ -147,9 +147,6 @@ static struct name_address_pair GLX_functions[] = {
    { "glXAssociateDMPbufferSGIX", (__GLXextFuncPtr) glXAssociateDMPbufferSGIX },
 #endif
 
-   /*** GLX_SGIX_swap_group ***/
-   { "glXJoinSwapGroupSGIX", (__GLXextFuncPtr) glXJoinSwapGroupSGIX },
-
    /*** GLX_SUN_get_transparent_index ***/
    { "glXGetTransparentIndexSUN", (__GLXextFuncPtr) glXGetTransparentIndexSUN },