fix(metadata): remove feature attribute and update dtype

- Removed feature attribute and its handling in __post_init__
- Changed dtype default from "dict" to None
- Adjusted __post_init__ to handle dtype "dict" case directly

src/biosets/features/metadata.py

@@ -15,19 +15,14 @@ class ValueWithMetadata(Value):
 class Metadata:
     """Metadata features that can be used to describe the dataset."""
 
-    feature: Any = None
-    id: Optional[str] = None
     dtype: str = "dict"
+    id: Optional[str] = None
     # Automatically constructed
     pa_type: ClassVar[Any] = None
     _type: str = field(default="Metadata", init=False, repr=False)
 
     def __post_init__(self):
-        if self.feature:
-            if isinstance(self.feature, Metadata):
-                self.pa_type = self.feature.pa_type
-                self.feature = self.feature.feature
-        elif self.dtype == "dict":
+        if self.dtype == "dict":
             self.pa_type = pa.struct(
                 {
                     k: string_to_arrow(v["dtype"])