Remove vue_broadcast_messages feature flag

What does this MR do and why?

Related: #385789 (closed)

Now that the Vue app has reached feature parity, we can safely remove the old HAML views. Remove the vue_broadcast_messages feature flag and all the old views along with it

Screenshots

This video shows off the basic workflow now that the feature is on by default. If you'd like to test the functionality against the HAML app, you'll need to use the master branch to do so. role_targeted_broadcast_messages is the other feature shown in the video, toggling it will show/hide the Target Roles setting for broadcast messages

How to set up and validate locally

1. Navigate to the broadcast messages view: http://gdk.test:3000/admin/broadcast_messages
2. Add/Update/Delete messages to test functionality
3. Test with role_targeted_broadcast_messages toggled on/off for full coverage

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.