st/mesa: remove an incorrect assertion

There is really no reason why the current DrawBuffer needs to be complete
at this point. In particular, the assertion gets hit on the X server side
in libglx when running .../piglit/bin/glx-get-current-display-ext -auto
(which uses indirect GLX rendering).

Fixes: 19b61799e3 ("st/mesa: don't cast the incomplete framebufer to st_framebuffer")
Reported-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/state_tracker/st_manager.c

@@ -837,8 +837,6 @@ st_manager_flush_frontbuffer(struct st_context *st)
    struct st_framebuffer *stfb = st_ws_framebuffer(st->ctx->DrawBuffer);
    struct st_renderbuffer *strb = NULL;
 
-   assert(st->ctx->DrawBuffer != _mesa_get_incomplete_framebuffer());
-
    if (stfb)
       strb = st_renderbuffer(stfb->Base.Attachment[BUFFER_FRONT_LEFT].Renderbuffer);
    if (!strb)