Refactor: removed model_map and it's imports.

Since, the reason of removing of model_map is explained in previous commits, so
removed those dependecies inside hallucination_model too.

deepeval/models/hallucination_model.py

@@ -2,17 +2,14 @@
 from deepeval.singleton import Singleton
 from sentence_transformers import CrossEncoder
 from deepeval.progress_context import progress_context
-from deepeval.models.model_map import model_map, name_to_card
-
 
 class HallucinationModel(metaclass=Singleton):
-    def __init__(self, model_name: str = "vectara-hallucination"):
+    def __init__(self, model_name: str = "vectara/hallucination_evaluation_model"):
         # We use a smple cross encoder model
         os.environ["TOKENIZERS_PARALLELISM"] = "false"
 
         # TODO: add this progress context in the correct place
         with progress_context(
             "Downloading HallucinationEvaluationModel (may take up to 2 minutes if running for the first time)..."
         ):
-            model_name = name_to_card(model_name)
             self.model = CrossEncoder(model_name)