Revert "gstlal_peakfinder_top.h Add FIXME to delete unused variable eventually"

This reverts commit 5bfbc124.

I realized gstlal_inspiral wasn't the only code using the peak_finder,
so removed my FIXME

gstlal/lib/gstlal/gstlal_peakfinder_top.h

@@ -47,7 +47,6 @@ struct gstlal_peak_state {
 	/* Interpolator */
 	LanczosTriggerInterpolant *interp;
 
-	/* FIXME I think is_gap is unused, we should delete it */
 	gboolean is_gap;
 	gboolean no_peaks_past_threshold;
 };