Revert "Add union decoding"

This reverts commit f8eb3cc.

Author: chardoncs

python/cocoindex/convert.py

@@ -90,9 +90,6 @@ def decode(value):
         field_path.pop()
         return decode
 
-    if src_type_kind == 'Union':
-        return lambda value: value.elem_type | None if value.nullable else value.elem_type
-
     if src_type_kind == 'Uuid':
         return lambda value: uuid.UUID(bytes=value)
 
@@ -171,4 +168,4 @@ def dump_engine_object(v: Any) -> Any:
         return [dump_engine_object(item) for item in v]
     elif isinstance(v, dict):
         return {k: dump_engine_object(v) for k, v in v.items()}
-    return v
+    return v