SubscriptionManager: Fix contradictionary JavaDoc

This function DOES require the synchronization which is already
mentioned in part of the JavaDoc: The synchronizeSubscriberByFCP() which
it calls will read the database of the WebOfTrust, so the caller has to
synchronize upon it.
Thus, this commit removes the opposite JavaDoc which says that no sync
is needed.
Merge branch 'master' into use-plugin-fcp-rewrite