Remove greet.yml action (#37587)

Summary:
This (`greet.yml`) action requires some _changes_ to "repo" settings which are _out of maintainers' controls_.
So, instead of wasting compute; let's just delete this action :(
🤗🌏

## Changelog:

[GENERAL] [REMOVED] - [Actions] Remove `greet.yml` action

Pull Request resolved: #37587

Test Plan: - Not needed.

Reviewed By: cortinico

Differential Revision: D46275607

Pulled By: cipolleschi

fbshipit-source-id: 80880568cbae1158006445e078e638e4e375cb73