Revert "llvmpipe: Always return some fence in flush (v2)"

Revert "llvmpipe: Always return some fence in flush (v2)"

This reverts commit f6a6da8131383d8eeee07cd59326a70f4b15866b.

With this commit we see massive amounts of asserts triggering
in lp_fence_wait(), assert(f->issued), for instance with libgl_xlib
state tracker and piglit. Not entirely sure if the assert could
just be removed.