Remove dependency management for org.webjars:hal-browser

We no longer make any use of the dependency and Spring Data REST has replaced its `spring-data-rest-hal-browser` module with `spring-data-rest-hal-explorer` which uses `org.webjars:hal-explorer`. As such we should stop having an opinion about the version of `org.webjars:hal-browser` and allow users to select whatever version suits their needs.