Remove python_repl from _BASE_TOOLS (#13962)

### **Description:**

Previously `python_repl` was a built-in tool, but now it has been moved
to `langchain_experimental`.

When I use `load_tools` I get an error:

```python
In [1]: from langchain.agents import load_tools

In [2]: load_tools(["python_repl"])
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
Cell In[2], line 1
----> 1 load_tools(["python_repl"])

File ~/workspace/langchain/libs/langchain/langchain/agents/load_tools.py:530, in load_tools(tool_names, llm, callbacks, **kwargs)
    528     tool_names.extend(requests_method_tools)
    529 elif name in _BASE_TOOLS:
--> 530     tools.append(_BASE_TOOLS[name]())
    531 elif name in _LLM_TOOLS:
    532     if llm is None:

File ~/workspace/langchain/libs/langchain/langchain/agents/load_tools.py:84, in _get_python_repl()
     83 def _get_python_repl() -> BaseTool:
---> 84     raise ImportError(
     85         "This tool has been moved to langchain experiment. "
     86         "This tool has access to a python REPL. "
     87         "For best practices make sure to sandbox this tool. "
     88         "Read https://github.com/langchain-ai/langchain/blob/master/SECURITY.md "
     89         "To keep using this code as is, install langchain experimental and "
     90         "update relevant imports replacing 'langchain' with 'langchain_experimental'"
     91     )

ImportError: This tool has been moved to langchain experiment. This tool has access to a python REPL. For best practices make sure to sandbox this tool. Read https://github.com/langchain-ai/langchain/blob/master/SECURITY.md To keep using this code as is, install langchain experimental and update relevant imports replacing 'langchain' with 'langchain_experimental'
```

In this case, it will be very confusing. I think it is no longer a
built-in tool now, so it can be removed from `_BASE_TOOLS`

### **Issue:** 

https://github.com/langchain-ai/langchain/issues/13858,
https://github.com/langchain-ai/langchain/issues/13859,
https://github.com/langchain-ai/langchain/issues/13856
### **Twitter handle:** 

[lin_bob57617](https://twitter.com/lin_bob57617)

libs/langchain/langchain/agents/load_tools.py

@@ -120,7 +120,6 @@ def _get_sleep() -> BaseTool:
 
 
 _BASE_TOOLS: Dict[str, Callable[[], BaseTool]] = {
-    "python_repl": _get_python_repl,
     "requests": _get_tools_requests_get,  # preserved for backwards compatibility
     "requests_get": _get_tools_requests_get,
     "requests_post": _get_tools_requests_post,