Remove drush option <contrib-destination> from "prepare install" command

We should parametrize this option through drush's make/yml files.

This will avoid an error that occurs in some situations where the folder
to specified in contrib-destination gets overwritten by a git clone.

e.g. : profiles/<my-profile> gets overwritten by a git clone while it might
have been parent folder of contrib modules previously downloaded.

This depends on the order a drush make/yml file has been written as far as
I can understand the issue.