chore:remove .env.example duplicate key (#5853)

docker/.env.example

 # The port on which the sandbox service runs
 SANDBOX_PORT=8194
 
-# ------------------------------
-# Environment Variables for qdrant Service
-# (only used when VECTOR_STORE is qdrant)
-# ------------------------------
-QDRANT_API_KEY=difyai123456
-
 # ------------------------------
 # Environment Variables for weaviate Service
 # (only used when VECTOR_STORE is weaviate)