Remove project and group vulnerability management feature flags

What does this MR do and why?

Related #513017 (closed)

Related #513024 (closed)

This removes the project- and group-level feature flags for the vulnerability management policy type:

1. vulnerability_management_policy_type
2. vulnerability_management_policy_type_group

References

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Before	After

How to set up and validate locally

You can find the vulnerability management policy by going to

1. Go to a project / group (respectively the vulnerability_management_policy_type and vulnerability_management_policy_type_group feature flags)
2. Go to Secure > Policies
3. Click "New policy", you can see the vulnerability management policy type tile and from there go to the editor