Removed .github/stale.yml as we're now using the Github Action

Summary:
As the title says, probot/stale is really unpredictable and happens to spam
a lot of issues, making hard for us to follow up on those that are really
relevant.

We're now using actions/stale and this file is no longer necessary.

Changelog:
[Internal] [Changed] - Removed .github/stale.yml as we're now using the Github Action

Reviewed By: mdvacca

Differential Revision: D33585581

fbshipit-source-id: 02a645ca98985edcee22d24c1ad4c0b7056fc15d