Remove network policies vuex store

What does this MR do and why?

Remove network policies vuex store

• it is no longer used with the removal of network policies

Related to #352285 (closed)

Screenshots or screen recordings

No visual changes. All pages work as expected

How to set up and validate locally

1. Enable :group_level_security_policies feature flag
2. Upload a GitLab Ultimate license
3. Navigate to a group => Security & Compliance => Policies => New policy and ensure everything works

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.