[Chore] Deprecate SupportsMultiModal.merge_by_field_config (#30170)

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

vllm/model_executor/models/blip2.py

@@ -523,8 +523,6 @@ class Blip2MultiModalProcessor(BaseMultiModalProcessor[Blip2ProcessingInfo]):
 class Blip2ForConditionalGeneration(
     nn.Module, SupportsMultiModal, SupportsPP, SupportsQuant
 ):
-    merge_by_field_config = True
-
     @classmethod
     def get_placeholder_str(cls, modality: str, i: int) -> str | None:
         if modality.startswith("image"):