[NEXUS-6747] Replace usage of restlet RepositoryUrlBuilder
[NEXUS-6747] Use base url set in general settings if available
Drop superfluous check on base url returned by BaseUrlHolder
No need to use GlobalRestApiSettings as BaseUrlHolder already takes care of it
Base url never ends up with '/'
Merge pull request #658 from sonatype/NEXUS-6747-repo-url-builder

REVIEW: [NEXUS-6747] Replace usage of restlet RepositoryUrlBuilder