Remove HTML todolist functionality

What does this MR do?

Follow up from #228846 (closed)

We have now removed HTML from all translations and these have been synced with Crowdin. This means we no longer need the HTML todolist built into the PO Linter and we should remove this so that no more strings can be added to this list.

1. Remove the HTML todolist
2. Remove the todolist functionality from the PO Linter
3. Remove the locale/en/* files.

Removing /locale/en/gitlab.po

This is not being synced with Crowdin and causes an issue with the gettext:lint task, as one of these old strings has angle brackets in it and these are not being removed in the Crowdin syncs.

I believe this is safe to remove because there are no translations in this file (only keys) and there are very few keys, all of which have not been updated in three years.

locale/en/gitlab.po  # contains  1,226 lines, with keys that have not been updated since 2017
locale/gitlab.pot    # contains 33,475 lines, with keys in english

I think what's been happening is, when the user has en as their locale, the locale/en file will be searched first -- in the vast majority of cases the key does not exist in that file, so it falls back to using the key itself as the translation (which is in English anyway). In the cases where the key exists and there's a translation, the translation would be taken from the en/gitlab.po file. This would actually be an issue as this locale is not being synced with Crowdin, so it we would never update it.

By removing this file, we skip the lookup in the locale/en/gitlab.po file and just go straight to using the key as the translation.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• ~~Label as security and @ mention @gitlab-com/gl-security/appsec~~
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team