mesa: Eliminate gl_config::rgbMode

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/main/context.c

@@ -287,7 +287,6 @@ _mesa_initialize_visual( struct gl_config *vis,
    assert(accumBlueBits >= 0);
    assert(accumAlphaBits >= 0);
 
-   vis->rgbMode          = GL_TRUE;
    vis->doubleBufferMode = dbFlag;
    vis->stereoMode       = stereoFlag;