Remove discord webhooks from environment variables

What does this MR do and why?

Remove discord webhooks from environment variables

The webhook on Discord has been deleted by Wick. To reduce the potential impact, the webhooks are removed from the environment variables, but can be added back if we decide to re-enable the feature and replaced the webhook urls with new ones in the project settings for ci variables.

Discord reference: https://canary.discord.com/channels/778180511088640070/1130474282791739404/1184541761482264637

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-policies-with-a-dry-run on how to perform dry-runs for new policies.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/doc/reactive/best_practices.md#use-the-sandbox-to-test-new-processors on how to make sure a new processor can be tested.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review