refactor(ng-lib): change cleanDups to private

[geromegrignon]

change cleanDups to private as it's an implementation detail

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/scullyio/scully/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Other... Please describe:

What is the current behavior?

cleanDups is publicly accessible but meant to be an implementation detail as stated by @SanderElias on Gitter :

the cleandups is private, it is an implementation detail. (as content-routes can have multiple instances this makes sure they are not leaking into the app, wreaking havoc on lists)

What is the new behavior?

cleanDups is no longer accessible

Does this PR introduce a breaking change?

• Yes
• No

[SanderElias]

LGTM