The only instance of unique visitors I could find when processing was disabled was in VisitsSummary.get, and I noticed the metric is enabled everywhere in this case. This removes the metric everywhere if processing is disabled.
Note: this will also remove the metric even if there is existing data. So if a user computes it for months initially, then disables it for months, the old data won't be visible. Not sure if this is desired (cc @tsteur/@mattab).