Remove dev feature flag from GET /github/callback API

[samarpan1738]

Issue Description

This API has been live on prod for a long time but the feature flag has not been removed.
Please verify if this needs to be removed or not

Reason for adding dev FF

1. dev FF is being checked in the redirect url
2. If dev=true , do not redirect . PR link

Usages in other repos
None

This is a subtask of #2075

Expected Behavior

Current Behavior

Screenshots

NA

Reproducibility

• This issue is reproducible
• This issue is not reproducible

Steps to Reproduce

NA

Severity/Priority

• Critical
• High
• Medium
• Low

Additional Information

Checklist

• I have read and followed the project's code of conduct.
• I have searched for similar issues before creating this one.
• I have provided all the necessary information to understand and reproduce the issue.
• I am willing to contribute to the resolution of this issue.

[samarpan1738]

As discussed with @prakashchoudhary07 we aren't sure about the exact reason for handling the dev FF here.
Hence not removing this for now