remove duplicate key in .env file

[MoustafaAMahmoud]

Motivation and Context

Removed the duplicate keys for AZURE_COSMOSDB_MONGO_VCORE_INDEX

Description

Contribution Checklist

• [x] I have built and tested the code locally and in a deployed app
• [x] For frontend changes, I have pulled the latest code from main, built the frontend, and committed all static files.
• [x] This is a change for all users of this app. No code or asset is specific to my use case or my organization.
• [x] I didn't break any existing functionality :smile: