Revert "zink: always force flushes when originating from api frontend"

This reverts commit 03f049f497.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27560>

src/gallium/drivers/zink/zink_context.c

@@ -3857,8 +3857,7 @@ zink_flush(struct pipe_context *pctx,
       }
    }
 
-   /* TODO: if swapchains gain timeline semaphore semantics, `flags` can be eliminated and no-op fence can return timeline id */
-   if (!batch->has_work && flags) {
+   if (!batch->has_work) {
        if (pfence) {
           /* reuse last fence */
           fence = ctx->last_fence;