Remove misleading step from "Allow force push" instructions

What does this MR do?

The implication of the step I'm removing is that when Code owner approvals is enabled, it prevents a subset of users from pushing changes to files with code owners, who would otherwise be able to do so. However, this is not the case.

The infotip next to Code owner approvals says (correctly) that the setting does not prevent users allowed to push from doing so on files with code owners. That means that code owner approval only applies to users who aren't allowed to push to the branch anyway.

It seems like the line I've removed from the docs in this MR is a relic of old behavior of the feature that isn't true anymore, unless there's an edge-case I'm missing.

Impetus was this Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/494202

cc @ashmckenzie to confirm that my understanding here is correct? (Sorry to single you out Ash, you were just the first Backend Engineer listed under the Source Code group 😅)

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