Remove outdated features

[enricobattocchi]

Describe the feature you'd like

We should revert or hide:

• PC-579 Add feature toggle for the new settings UI #163
• PC-581 indexables page feature toggle #164

[enricobattocchi]

2 things:

1. See https://yoast.slack.com/archives/C03KU0EHCNQ/p1675771147990779?thread_ts=1675694734.622359&cid=C03KU0EHCNQ so we want to keep the threshold part, but hidden
2. that section wasn't actually removed in the PR, but made visible since the check on the flag was removed.

I'd reinstate the check on the flag then

[enricobattocchi]

So we need to reinstate the indexables page feature toggle and only remove the settings UI toggle instead for now.
We'll revise this when the indexables page is removed from Free.

[enricobattocchi]

We decided to remove the Indexables Page from Free, so we're back to the removal in the Test Helper as well.