Revert "feat: build codegen on postinstall (#46227)"

[Saadnajmi]

This reverts commit 0cb97f0.

Summary:

Revert this commit that adds a post install script for a couple of reasons:

1. (EDIT: This turns out to be unrelated) The postinstall script causes yarn install to fail on React Native macOS, where we use Yarn 4. I'm not entirely sure why, but I probably won't debug it for the rest of the reasons.
2. postinstall scripts (at least inside Microsoft) are viewed as a security risk. Any package in your dependency tree can get compromised, add the phase, and run arbitrary code. This has happened in the past with React Native past if I recall correctly. As such, we disable postinstall scripts in many of our repos (including rnx-kit and react-native-test-app).
3. The issue this is trying to solve is to help newcomers avoid a stale cache when they switch branches in the React Native monorepo and only run yarn install. I think it would be sufficient to add some documentation somewhere that it is expected one runs yarn && yarn build to use this repo locally? That's a fairly common practice in monorepos, at least ones inside Microsoft.

Changelog:

[INTERNAL] [SECURITY] - Remove post install script phase in the React Native monorepo

Test Plan:

CI should pass

[react-native-bot]

	Warnings
⚠️	🔒 package.json - Changes were made to package.json. This will require a manual import by a Facebook employee.

Generated by 🚫 dangerJS against 7d40d69

[facebook-github-bot]

@huntie has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@huntie merged this pull request in 8ac80e3.

[react-native-bot]

This pull request was successfully merged by @Saadnajmi in 8ac80e3

^{When will my fix make it into a release? | How to file a pick request?}