mesa: remove ctx->Const.sRGBCapable

It always had the same value as ctx->Extensions.EXT_framebuffer_sRGB.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>

src/mesa/state_tracker/st_extensions.c

@@ -425,7 +425,6 @@ void st_init_extensions(struct st_context *st)
                                    PIPE_TEXTURE_2D, 0,
                                    PIPE_BIND_RENDER_TARGET)) {
          ctx->Extensions.EXT_framebuffer_sRGB = GL_TRUE;
-         ctx->Const.sRGBCapable = GL_TRUE;
       }
    }