Revert "New projects don't automatically inherit from group-level"

What does this MR do and why?

We deployed a fix for projects not inheriting Slack integration settings, but it introduced a regression on self-managed instances.

When Slack integration is enabled at the instance level, project creation fails entirely.

Impact:

Self-managed instances with instance-level Slack integration cannot create new projects.

Current workaround:

Disable Slack integration at instance level.

Resolution:

This MR reverts the problematic changes to restore project creation functionality. We'll implement a proper fix that addresses the inheritance issue.

Related to #461056 (closed)