revert(core): "applied secrets_map in load to plain string values" (#33913)

Reverts #33678

Breaking API change

Author: mdrxy

libs/core/langchain_core/load/load.py

@@ -265,8 +265,6 @@ def _load(obj: Any) -> Any:
             return reviver(loaded_obj)
         if isinstance(obj, list):
             return [_load(o) for o in obj]
-        if isinstance(obj, str) and obj in reviver.secrets_map:
-            return reviver.secrets_map[obj]
         return obj
 
     return _load(obj)