Removed the definition of the duplicate index on the id column and re… (#167)

Removed the definition of the duplicate index on the id column and
removed the column attributes that do not match what PostgreSQL uses for
a primary key. A primary key always has an index and is always not
nullable.

Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

Author: jim-mlodgenski

langchain_postgres/vectorstores.py

@@ -201,7 +201,7 @@ class EmbeddingStore(Base):
         __tablename__ = "langchain_pg_embedding"
 
         id = sqlalchemy.Column(
-            sqlalchemy.String, nullable=True, primary_key=True, index=True, unique=True
+            sqlalchemy.String, primary_key=True
         )
 
         collection_id = sqlalchemy.Column(