Remove notification content extension if not used

[guarani]

There's a notification content extension in the project that doesn't appear to be used. My first thought was that it was used for long-press previews of rich text (images, gifs, etc) in push notifications. @hassaanelgarem pointed out in #19526 (comment) that it's actually probably not used.

We should remove it if it's not used to simplify the Xcode project.

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