qa: remove tests covered by backend integration/unit tests

Created by: flying-robot

Normal users were previously blocked from changing metadata like displayName or avatarURL if auth.enableUsernameChanges was false[0]. This was because the UI always includes the username value in its payload, which triggered the check incorrectly on the backend and blocked an action that should have succeeded.

The frontend suite covers cases[1] like hand-crafted GraphQL requests that could potentially trigger the site configuration error, so this PR removes the extra tests and renumbers the remainder.

ref https://github.com/sourcegraph/sourcegraph/issues/15877

[0] https://github.com/sourcegraph/sourcegraph/pull/15693 [1] https://github.com/sourcegraph/sourcegraph/issues/15877#issuecomment-729402084