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 https://github.com/wordpress-mobile/WordPress-iOS/pull/19526#discussion_r1008375586 that it's actually probably not used.

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

[peril-wordpress-mobile[bot]]

	Fails
:no_entry_sign:	Please add a feature label to this issue. e.g. 'Stats'

Generated by :no_entry_sign: dangerJS

[peril-wordpress-mobile[bot]]

	Fails
:no_entry_sign:	Please add a feature label to this issue. e.g. 'Stats'

Generated by :no_entry_sign: dangerJS

[peril-wordpress-mobile[bot]]

	Fails
:no_entry_sign:	Please add a feature label to this issue. e.g. 'Stats'

Generated by :no_entry_sign: dangerJS