Do not clear delivered notifications on app activate

[polo2187]

Fixes #6320

First time contributor checklist

• I have read the README and CONTRIBUTING documents
• I have signed the Contributor Licence Agreement

Contributor checklist

• My commits are rebased on the latest main branch
• My commits are in nice logical chunks
• My contribution is fully baked and is ready to be merged as is
• I have tested my contribution on these devices:

• iPhone 15 Pro, iOS 26.6

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Description

Signal iOS was clearing delivered notifications from Apple Notification Center when the app was opened.

This change stops removing delivered notifications in clearNotificationsForAppActivate() and only clears pending notifications there.

Delivered notifications should remain visible until the related conversation is actually opened/read and removed through the existing thread/message-specific cancellation paths.

Testing

• Receive one or more Signal messages.
• Confirm notifications are visible in Notification Center.
• Open Signal without opening the related conversation.
• Verify notifications remain visible in Notification Center.
• Open the conversation.
• Verify the corresponding notifications are dismissed through existing cancellation logic.