[V0 Deprecation] Remove placeholder attn (#25510)

Signed-off-by: Thomas Parnell <tpa@zurich.ibm.com>

vllm/attention/selector.py

@@ -142,7 +142,6 @@ def get_attn_backend(
     dtype: torch.dtype,
     kv_cache_dtype: Optional[str],
     block_size: int,
-    is_attention_free: bool = False,
     use_mla: bool = False,
     has_sink: bool = False,
 ) -> type[AttentionBackend]:
@@ -156,7 +155,6 @@ def get_attn_backend(
         dtype=dtype,
         kv_cache_dtype=kv_cache_dtype,
         block_size=block_size,
-        is_attention_free=is_attention_free,
         use_v1=envs.VLLM_USE_V1,
         use_mla=use_mla,
         has_sink=has_sink,
@@ -169,17 +167,10 @@ def _cached_get_attn_backend(
     dtype: torch.dtype,
     kv_cache_dtype: Optional[str],
     block_size: int,
-    is_attention_free: bool,
     use_v1: bool = False,
     use_mla: bool = False,
     has_sink: bool = False,
 ) -> type[AttentionBackend]:
-    # If there are no attention layers (e.g. we are running Mamba),
-    # use the placeholder NO_ATTENTION
-    if is_attention_free:
-        from vllm.attention.backends.placeholder_attn import (
-            PlaceholderAttentionBackend)
-        return PlaceholderAttentionBackend
 
     # Check whether a particular choice of backend was
     # previously forced.