Fixed hardcoded "vector" and replaced with vector_query_field variable (#12126)

- **Description:** In the max_marginal_relevance_search function of the
ElasticsearchStore vector store, the name of the field corresponding to
the vector embedding of the document is hard coded in the delete
statement that drops the field from the document metadata. This results
in an exception if the vector embedding field is customized. This PR
changes the hard-coded "vector" into the vector_query_field variable.
  - **Issue:** None
  - **Dependencies:** None
  - **Tag maintainer:** @hwchase17

Co-authored-by: Shilong Dai <sdai@viperfish.net>

libs/langchain/langchain/vectorstores/elasticsearch.py

@@ -662,7 +662,7 @@ class ElasticsearchStore(VectorStore):
 
         if remove_vector_query_field_from_metadata:
             for doc in selected_docs:
-                del doc.metadata["vector"]
+                del doc.metadata[self.vector_query_field]
 
         return selected_docs