docs: remove unused imports in Conversational RAG tutorial (#25297)

Cleaned up the "Tying it Together" section of the Conversational RAG
tutorial by removing unnecessary imports that were not used. This
reduces confusion and makes the code more concise.

Thank you for contributing to LangChain!

PR title: docs: remove unused imports in Conversational RAG tutorial

PR message:

Description: Removed unnecessary imports from the "Tying it Together"
section of the Conversational RAG tutorial. These imports were not used
in the code and created confusion. The updated code is now more concise
and easier to understand.
Issue: N/A
Dependencies: None
LinkedIn handle: [Hassan
Memon](https://www.linkedin.com/in/hassan-memon-a109b3257/)
Add tests and docs:

Hi [LangChain Team Member’s Name],

I hope you're doing well! I’m thrilled to share that I recently made my
second contribution to the LangChain project. If possible, could you
give me a shoutout on LinkedIn? It would mean a lot to me and could help
inspire others to contribute to the community as well.

Here’s my LinkedIn profile: [Hassan
Memon](https://www.linkedin.com/in/hassan-memon-a109b3257/).

Thank you so much for your support and for creating such a great
platform for learning and collaboration. I'm looking forward to
contributing more in the future!

Best regards,
Hassan Memon

docs/docs/tutorials/qa_chat_history.ipynb

@@ -1012,14 +1012,9 @@
    "outputs": [],
    "source": [
     "import bs4\n",
-    "from langchain.agents import AgentExecutor, create_tool_calling_agent\n",
     "from langchain.tools.retriever import create_retriever_tool\n",
     "from langchain_chroma import Chroma\n",
-    "from langchain_community.chat_message_histories import ChatMessageHistory\n",
     "from langchain_community.document_loaders import WebBaseLoader\n",
-    "from langchain_core.chat_history import BaseChatMessageHistory\n",
-    "from langchain_core.prompts import ChatPromptTemplate, MessagesPlaceholder\n",
-    "from langchain_core.runnables.history import RunnableWithMessageHistory\n",
     "from langchain_openai import ChatOpenAI, OpenAIEmbeddings\n",
     "from langchain_text_splitters import RecursiveCharacterTextSplitter\n",
     "from langgraph.checkpoint.memory import MemorySaver\n",