folks-sync-only: Drop testcase, obsolete

This testcase is meant to test a custom libfolks feature we carried downstream Aand that was proposed upstream and WONTFIX'ed in https://bugzilla.gnome.org/show_bug.cgi?id=741643

We're thus considering that this testcase does not bring any value to the project and is rather an hindrance since:

• the actual use-case for the feature is not really documented
• the implementation is problematic and has major architectural issues
• a proper solution would involve doing it in EDS rather than libfolks

No replacement is planned since contacts management via libfolks is not in the current priorities.