DNN-6024 Don't default link manager to https:// when SSL is enabled

[DNN-6024](https://dnntracker.atlassian.net/browse/DNN-6024) Don't default link manager to https:// when SSL is enabled. Instead, update the `AbsoluteToRelativeUrls` method to ignore the protocol.

Reverts some of 9703902883c289717e9ea926fd7e5f6610e2a8e2