Let the user hide messages that have been removed

[hayaksi1]

Content

Every removed message leaves a "Message removed" placeholder behind, and a room that has been moderated heavily ends up with more placeholders than messages. A new switch in Advanced settings, on by default, drops those items from the timeline instead.

The preference sits next to the other per-session timeline rendering options and is read the same way as the read receipt one, so the timeline rebuilds as soon as it is changed rather than on the next launch.

The pinned message list is unaffected: it asks for removed messages explicitly, so a pinned event that was later removed still explains why it is no longer there.

Motivation and context

Relates to #4417.

Tests

• TimelineItemsFactoryTest — with the preference off, a removed event is left out of the timeline and the message next to it is kept.
• The existing :features:messages:impl, :features:preferences:impl and :libraries:preferences:impl suites pass unchanged.

These do not cover what happens to a day separator whose only event was removed; the separator is still produced, as it is today for any hidden item.

Tested devices

• Physical
• Emulator
• OS version(s): API 36

Checklist

• I am aware of the etiquette.
• This PR was made with the help of AI:
  • Yes. In this case, please request a review by Copilot.
  • No.
• Changes have been tested on an Android device or Android emulator with API 24
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request title will be used in the release note, it clearly defines what will change for the user
• Pull request includes screenshots or videos if containing UI changes
• You've made a self review of your PR

[github-actions]

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