Remove PR list generation at code freeze

[loremattei]

This PR removes the step in Fastlane's code_freeze lane that generates a text file with a list of the PRs that made it to the new version. With the current process to generate the release notes and the new tooling we built to generate the draft of the release announcement, this file is not used anymore.

This is also removing the get_pullrequests_list lane.

Regression Notes

1. Potential unintended areas of impact
   --

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   --

3. What automated tests I added (or what prevented me from doing so)
   --

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

	Warnings
⚠️	This PR is assigned to a milestone which is closing in less than 4 days Please, make sure to get it merged by then or assign it to a later expiring milestone

Generated by 🚫 dangerJS

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APKs:

• Jetpack-pr-15564-build-116946.apk
• WordPress-pr-15564-build-116946.apk