Remove dependency credential test

[sophiewigmore]

• the change in CDN results in these tests failing, because the new CDN config does not support credentials in this way.
• we believe that consumers do not use this feature, as we have not had any reports of this causing failures for consumers.
• if we get reports that consumers cannot download dependencies from the CDN with credentials, and we deem this to be a necessary feature, we can update the CDN config and re-add these tests.
  cc @robdimsdale