Remove static webhook feature flags

What does this MR do and why?

This change removes two feature flags that were default_enabled: true:

• webhooks_static_snippet_hook_attrs
• webhooks_static_label_hook_attrs

Related issues:

• #440384
• gitlab-com/gl-infra/production#17798 (closed)
• gitlab-com/gl-infra/production#17566 (closed)

MR acceptance checklist

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Related to #440384