[Refactor] Remove duplicate create_opening route in dashboard router

[Jaswanth-006]

Description

The route for posting openings is defined twice:

1. In app/routers/dashboard.py as POST /faculty/opening
2. In app/routers/openings.py as POST / (under prefix /openings)

This duplication leads to code drift and redundant database queries.

Expected Behavior

The duplicate route in dashboard.py should be removed, and the frontend should use the unified openings router endpoint.