Clean up unusued group trigger files

What does this MR do and why?

Following the merge of the Vue webhook form !210618 (merged)

We should no longer need the haml templates for the ee- specific triggers, as they are now replaced by:

• ee/app/assets/javascripts/webhooks/components/vulnerability_events_trigger_item.vue
• ee/app/assets/javascripts/webhooks/components/group_events_trigger_items.vue

References

Screenshots or screen recordings

No UI changes

Before	After

How to set up and validate locally

1. For group webhooks, you'd need to have a premium or ultimate license.
2. Choose a top level group . Click settings -> webhooks -> Add a webhook.
   1. Attempt to change the triggers (fill out the form). Check vulnerability events, subgroup, project events
   2. Save
   3. You should see the created hook and 'chips' that reflect the triggers you saved
3. For project webhooks. Go to a Project settings page -> webhooks -> Add a webhook
   1. Repeat above steps

MR acceptance checklist

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Related to #580028