Remove default snapshot library

[fitzoh]

By default, skipper server instances have the spring libs-snapshot set as a maven repo.

https://github.com/spring-cloud/spring-cloud-skipper/blob/36be4df251ab5178a1dbd24737aae3831ddf0b45/spring-cloud-skipper-server-core/src/main/resources/application.yml#L135-L138

The main spring-cloud-skipper-server deployable should probably contain defaults suitable for a production deployment, which would mean removing the snapshot repo (and potentially the experimental package repo).

Those would probably make more sense to be included in a skipper server sample (possibly one created in #552)