[NXCM-5131] Fix itemMaxAge settings for non-maven repositories

This pull reverts the initial change for NXCM-5131, because it affected behavior of non-maven-repositories inheriting from AbstractProxyRepository. instead of the implicit default of itemMaxAge (1440 minutes), 0 would be used instead.

With this change, the repository resource sets itemMaxAge for repositories that do not have the maven-specific artifact/metadataMaxAge.