Remove the unused policy store REST update client

What does this MR do and why?

This MR removes updatePolicyStorePolicy from ee/app/assets/javascripts/api.js and its describe block from ee/spec/frontend/api_spec.js. That REST client method has no caller now that !255387 (merged) has merged, because the policy store frontend updates policies through the governPolicyUpdate GraphQL mutation instead.

Related to #617792

References

• Parent MR (stack base): !255387 (merged)
• Issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/617792 (confidential)
• Plan comment: https://gitlab.com/gitlab-org/gitlab/-/issues/617792#note_3830226456

Screenshots or screen recordings

No UI changes.

How to set up and validate locally

1. Check out this branch. It was stacked on !255387 (merged), which merged on 2026-09-16. The MR now targets master and was rebased onto it the same day.
2. Run yarn jest ee/spec/frontend/api_spec.js and confirm it passes.
3. Run rg updatePolicyStorePolicy ee/app ee/spec and confirm there are no matches.

Note: Api.getPolicyStorePolicy, buildPolicyStorePolicyUrl, and policyStorePolicyPath in the same file are also unused since the read migration. They were unused before this work and are left for a separate cleanup.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist.

🤖 Generated with Claude Code