[V1] Simplify prefix caching logic by removing num_evictable_computed_blocks

[heheda12345]

The evictable_computed_blocks are removed from self.free_block_queue during self._touch(computed_blocks), so we do not need to treat them specially in allocate_slots.

The num_evictable_computed_blocks is always zero (I've checked here 667af1b) because self._touch also increases the reference count by one.

The below command has no diff in performance.

VLLM_USE_V1=1 python3 benchmarks/benchmark_prefix_caching.py \
--model meta-llama/Meta-Llama-3-8B-Instruct  \
--num-prompts 50 \
--input-length-range 500:500  \
--prefix-len 250     --seed 0 --output-len 500

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[comaniac]

Ah good catch. I guess it was useful but became useless after some iterations of refactoring...