fix liip cdn url generating (don't use cdn prefix twice)

[AlexKovalevych]

Fix which prevents liip service from using cdn prefix twice (second time its used here https://github.com/sonata-project/SonataMediaBundle/blob/master/Provider/ImageProvider.php#L153 during image url creating)

[core23]

Are you still working on this? Please squash and rebase or close this PR @AlexKovalevych

[soullivaneuh]

According to the new Sonata version management and next major release plan, this project has been refactored regarding branching and versioning.

If you see this message, your PR concerns a patch or a minor release and is not targeting the right branch.

So I'm closing this one, but don't see it as a refusal. If you think your work is still relevant and want to continue, feel free to reopen it on the right branch (e.g. the default one).

Regards.