Remove the PIN lock feature from the app

[maxme]

Remove it from the app and remove the settings in App Settings.

Before we remove it, we should:

1. One version before the removal, we should alert PIN lock users (via a dialog on startup) that this feature will be removed, and propose them a better way to share their phone (using a Guest account on their phone). Add a page in the Helpshift FAQ and have a shortcut to open it.
2. On upgrade, show the same dialog for the users who still have the feature enabled, and add a "contact us" button that redirects to helpshift.

[maxme]

Relevant: https://support.google.com/pixelphone/answer/6115141

[daniloercoli]

We should also mark the library as deprecated / not supported anymore on the relevant gitHub repo.

[stale]

This issue has been marked as stale because:

• It has been inactive for the past year.
• It isn't in a project or a milestone.
• It hasn’t been labeled [Pri] Blocker, [Pri] High, or good first issue.

Please comment with an update if you believe this issue is still valid or if it can be closed. This issue will also be reviewed for validity and priority (cc @designsimply).

[rachelmcr]

The pin lock was removed in #8236 and the README for https://github.com/wordpress-mobile/PasscodeLock-Android notes that it is not maintained. Closing as resolved.