NO-ISSUE: Removed OKD_RPMS in README, is no longer necessary

[fiblan]

OKD_RPMS setting is no longer necessary as assisted-service now uses okd-rpms image from the release image (v. #4922)

List all the issues related to this PR

• [ ] New Feature
• [ ] Enhancement
• [ ] Bug fix
• [ ] Tests
• [x] Documentation
• [ ] CI/CD

What environments does this code impact?

• [ ] Automation (CI, tools, etc)
• [ ] Cloud
• [ ] Operator Managed Deployments
• [x] None

How was this code tested?

• [ ] assisted-test-infra environment
• [ ] dev-scripts environment
• [ ] Reviewer's test appreciated
• [ ] Waiting for CI to do a full test run
• [ ] Manual (Elaborate on how it was tested)
• [x] No tests needed

Checklist

• [ ] Title and description added to both, commit and PR.
• [ ] Relevant issues have been associated (see CONTRIBUTING guide)
• [ ] This change does not require a documentation update (docstring, docs, README, etc)
• [ ] Does this change include unit-tests (note that code changes require unit-tests)

Reviewers Checklist

• Are the title and description (in both PR and commit) meaningful and clear?
• Is there a bug required (and linked) for this change?
• Should this PR be backported?