[Misc] remove unused file (cache.py) (#2377)

### What this PR does / why we need it?
cache.py only contains a function that will never be called, so remove
it.

### Does this PR introduce _any_ user-facing change?
No

- vLLM version: v0.10.0
- vLLM main:
vllm-project/vllm@f1f0d2f

Signed-off-by: 22dimensions <[email protected]>

Author: 22dimensions