build: remove outdated/obsolete github PR utility scripts (angular#45868)

Removes scripts and documentation for tools which are no longer
actively used and likely do not work anyway at this point.

All of our PR-related tooling has moved into the `yarn ng-dev` command.
The `PR_REVIEW` markdown file is deleted since it was soley about
the legacy review scripts and the PR review workflow also does not
seem like the common/standard workflow Angular team members follow.

PR Close angular#45868

Author: devversion