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

https://issues.sonatype.org/browse/NEXUS-6747

Calculate repository content urls based on BaseUrlHolder instead using restlet RepositoryUrlBuilder, without taking into consideration the "force base url" as that functionality will be dropped in the future (https://issues.sonatype.org/browse/NEXUS-6490 https://issues.sonatype.org/browse/NEXUS-6373)
can drop baseUrl.endsWith check, the value from BaseUrlHolder.get() will *never* end with "/" so you can "$baseUrl/content/..."