libweston: Remove runtime render switching

It is only enabled by a debug key binding, currently not tested at all,
and is seems it doesn't really work, so let's remove it. This also
removes it from the man page.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

libweston/renderer-gl/gl-renderer.c

@@ -1011,9 +1011,6 @@ draw_paint_node(struct weston_paint_node *pnode,
 	GLint filter;
 	struct gl_shader_config sconf;
 
-	/* In case of a runtime switch of renderers, we may not have received
-	 * an attach for this surface since the switch. In that case we don't
-	 * have a valid buffer or a proper shader set up so skip rendering. */
 	if (gb->shader_variant == SHADER_VARIANT_NONE &&
 	    !buffer->direct_display)
 		return;