docs: Remove duplicated line in Get Started section (#19182)

Line `from langchain_openai import ChatOpenAI` is put twice in Get
Started / Serving with LangServe section.
Imports on lines 559 and 566 are identical

Co-authored-by: Vitalii <vitalii@localhost>

docs/docs/get_started/quickstart.mdx

@@ -563,7 +563,6 @@ from langchain_community.vectorstores import FAISS
 from langchain_text_splitters import RecursiveCharacterTextSplitter
 from langchain.tools.retriever import create_retriever_tool
 from langchain_community.tools.tavily_search import TavilySearchResults
-from langchain_openai import ChatOpenAI
 from langchain import hub
 from langchain.agents import create_openai_functions_agent
 from langchain.agents import AgentExecutor