Unschedule group deletion on deleting user group membership removal

What does this MR do?

Since GitLab 12.8, on Premium/Silver or higher tiers, groups are deleted with a delayed interval. This is 7 days be default, and configurable in Admin Settings.

It can so happen that a group member schedules a group for deletion, and is removed from the group before the actual deletion happens.

As the deleting member no longer has access to the group, the deletion is scheduled to run everyday and never succeed.

This MR resolves the issue by removing the group deletion schedule when a group member is removed from the group (if they exist and were scheduled by the deleting member).

Mentions #39504 (closed)

Screenshots (strongly suggested)

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 #39504 (closed)