Fix webhook "custom" option appearing

What does this MR do and why?

Remove webhook "custom" option from list since it's not currently a true option.

Resolves #302 (closed)

Screenshots or screen recordings

How to set up and validate locally

• Press create new webhook
• Check provider options

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• Have you checked this doesn't interfere/conflict/duplicate someone elses work?
• Have you fully tested your changes?
• Have you resolved any lint issues?
• Have you assigned a reviewer?
• Have you applied correct labels?