Restricted Headers: remove ones that are allowed, but leave the mech for checking in place for future use

[kraftbj]

Partly reverts the functionality of #670.
Resolves #718

Per the conversation in #669, the presence of these headers—in and of themselves—do not create a third-party upgrader situation. The UpdateURI, which is already checked elsewhere, does prevent the updates from WordPress.org, but the headers originally included in this code are for information only.

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[afragen]

Thank you Brandon.

[sc0ttkclark]

This makes a lot of sense and keeps the functionality available for problematic comments that could be moved in here like the one which lets you override the Update URI (https://make.wordpress.org/core/2021/06/29/introducing-update-uri-plugin-header-in-wordpress-5-8/)

[danieliser]

I approve this! ☝️

[xwolfde]

Thank you!

[dd32]

I've chatted with @chriscct7 and I agree with reverting this change for now, assigning it to him to handle.

[lucprincen]

Thank you!