fix: Remove postinstall-postinstall

`postinstall-postinstall` is unnecessary for anything in this project, I'm unsure why it's even in `bluesky-social/social-app`. The switch to `pnpm` means this package will never do anything, as its setup process only ever uses `npm` or `yarn`. Furthermore, if `yarnpkg` is in path, it will fail to install modules as it attempts to run the package manager `yarn@pnpm@<version>`, which doesn't work.

This package is hardcoded to work in very specific environments that can be easily identified by looking at its [source code](https://github.com/ds300/postinstall-postinstall/blob/master/run.js), making it worse than useless for this project.