[V0 deprecation] Remove no longer used get_metadata_cls (#28370)

Signed-off-by: Lucas Wilkinson <lwilkins@redhat.com>

vllm/v1/attention/backends/triton_attn.py

@@ -10,7 +10,6 @@ import torch
 from vllm.attention.backends.abstract import (
     AttentionBackend,
     AttentionImpl,
-    AttentionMetadata,
     AttentionType,
     MultipleOf,
 )
@@ -176,10 +175,6 @@ class TritonAttentionBackend(AttentionBackend):
     def get_impl_cls() -> type["TritonAttentionImpl"]:
         return TritonAttentionImpl
 
-    @staticmethod
-    def get_metadata_cls() -> type["AttentionMetadata"]:
-        return TritonAttentionMetadata
-
     @staticmethod
     def get_kv_cache_shape(
         num_blocks: int,