Remove sub_group_webhooks feature flag

What does this MR do?

This MR removes the sub_group_webhooks feature flag.

The feature flag was introduced because there was no limit for the number of webhooks in a group. This raised a concern that enabling group hooks to be triggered from a sub-group could result in a large number of requests per action.

With !25129 (merged) we introduced a limit for webhooks on a group and the feature flag is not needed anymore

How to test in staging

1. Remove the feature flag by running /chatops run feature delete sub_group_webhooks --staging
2. Create a group.
3. Go to https://webhook.site/ to create a test webhook endpoint.
4. Create a new webhook under /groups/[your group]/-/hooks that points to the link you got from webhook.site and make it trigger on issue events.
5. Create a sub-group
6. Create a project in your subgroup
7. Create an issue in your project.
8. Check webhook.site to see if the webhook was triggered.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #207123 (closed)