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.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>