Clean up imports in short-term-memory.mdx (#1182)

## Overview
Removed unused import statements for AnyMessage and AgentState in
[dynamic prompt example
](https://docs.langchain.com/oss/python/langchain/short-term-memory#prompt)

## Type of change

**Type:** Fix formatting
## Checklist
<!-- Put an 'x' in all boxes that apply -->
- [x] I have read the [contributing guidelines](README.md)
- [ ] I have tested my changes locally using `docs dev`
- [x] All code examples have been tested and work correctly
- [ ] I have used **root relative** paths for internal links
- [ ] I have updated navigation in `src/docs.json` if needed
- I have gotten approval from the relevant reviewers
- (Internal team members only / optional) I have created a preview
deployment using the [Create Preview Branch
workflow](https://github.com/langchain-ai/docs/actions/workflows/create-preview-branch.yml)

## Additional notes
<!-- Any other information that would be helpful for reviewers -->

Author: donMichaelL

src/oss/langchain/short-term-memory.mdx

@@ -781,17 +781,15 @@ Access short term memory (state) in middleware to create dynamic prompts based o
 
 :::python
 ```python
-from langchain.messages import AnyMessage
-from langchain.agents import create_agent, AgentState
+from langchain.agents import create_agent
 from typing import TypedDict
+from langchain.agents.middleware import dynamic_prompt, ModelRequest
 
 
 class CustomContext(TypedDict):
     user_name: str
 
 
-from langchain.agents.middleware import dynamic_prompt, ModelRequest
-
 def get_weather(city: str) -> str:
     """Get the weather in a city."""
     return f"The weather in {city} is always sunny!"
@@ -817,6 +815,7 @@ result = agent.invoke(
 )
 for msg in result["messages"]:
     msg.pretty_print()
+
 ```
 
 ```shell title="Output"