[KVConnector] Remove v0-related kv connector components such as kv pipe and kv lookup buffer

[KuntaiDu]

Purpose

As v0 engine is now fully removed, it is time to remove v0-related kv connector components such as kv pipe and kv lookup buffer.

Test

1. Made sure that kv_lookup_buffer and kv_pipe no longer exists in vLLM codebase
2. Made sure that v1 connector is unaffected, by testing

vllm serve deepseek-ai/DeepSeek-V2-Lite --kv-offloading-backend lmcache --kv-offloading-size 100

and LMCache multi-processing connector.

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[chatgpt-codex-connector]

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[gemini-code-assist]

Code Review

This pull request removes a significant amount of code related to the now-obsolete v0 KV connector, including KV pipes and lookup buffers. The changes consist entirely of deleting unused files and their corresponding tests. This is an excellent cleanup effort that simplifies the codebase by removing legacy components. The tests described in the pull request description appear sufficient to verify that the v1 connector is unaffected by these changes. The deletions are appropriate and the pull request looks good to merge.