[MNT] Delete outdated add_pr2project.yml

[alyssadai]

• Related to Remove outdated add_pr2project.yml from repos workflows#119

FYI: The best way to find all instances of a file across all repos / files using an outdated PAT (e.g., NB_PROJECT_PAT) is using the search bar in https://github.com/neurobagel. Did not think of that the first time around when addressing neurobagel/workflows#119.... 🙈

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