[Misc] Further clean up chunked prefill and prefix caching init (#29186)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

vllm/config/cache.py

@@ -73,8 +73,8 @@ class CacheConfig:
     sliding_window: int | None = None
     """Sliding window size for the KV cache. This is primarily set in
     `ModelConfig` and that value should be manually duplicated here."""
-    enable_prefix_caching: bool | None = None
-    """Whether to enable prefix caching. Enabled by default for V1."""
+    enable_prefix_caching: bool = True
+    """Whether to enable prefix caching."""
     prefix_caching_hash_algo: PrefixCachingHashAlgo = "sha256"
     """Set the hash algorithm for prefix caching:\n
     - "sha256" uses Pickle for object serialization before hashing.\n