Remove default api url from blockfrost context

This fixes a bug where network and provided project_id mismatch.

Author: cffls

pycardano/backend/blockfrost.py

@@ -67,7 +67,7 @@ def __init__(
         self,
         project_id: str,
         network: Optional[Network] = None,
-        base_url: str = ApiUrls.preprod.value,
+        base_url: Optional[str] = None,
     ):
         if network is not None:
             warnings.warn(