[Docker] Remove flashinfer cache copy

[mmangkad]

Summary

Fix https://github.com/sgl-project/sglang/actions/runs/24320223003/job/71004907738#step:8:8520. After #22491 removed the manual flashinfer cubin download steps, /root/.cache/flashinfer is no longer populated.

cc @Kangyan-Zhou

[gemini-code-assist]

Code Review

This pull request updates the docker/Dockerfile to remove the instruction for copying the flashinfer cache from the flashinfer_cache stage, while retaining the copy of the jit-cache package. I have no feedback to provide.