Eliminate an unneeded extra WCS fit in ImageDifferenceTask

ImageDifferenceTask had a workaround for the fact that our
tan-sip fitter used to fail if CRPIX was large. That's been fixed,
so I removed the workaround and managed to eliminate use of
a hard-coded astrometry task at the same time.

python/lsst/pipe/tasks/imageDifference.py

@@ -576,19 +576,14 @@ def run(self, sensorRef):
             sources=diaSources,
         )
 
-    # One problem is that the SIP fits are w.r.t. CRPIX,
-    # and these coadd patches have the CRPIX of the entire
-    # tract, i.e. off the image.  This causes
-    # register.fitWcs to fail.  A workaround for now is to
-    # re-fit the Wcs which returns with a CRPIX that is on
-    # the image, and *then* to fit for the relative Wcs.
-    #
     def fitAstrometry(self, templateSources, templateExposure, selectSources):
-        """Fit the relative astrometry between templateSources and selectSources"""
-        sipOrder = self.config.templateSipOrder
-        astrometer = measAstrom.Astrometry(measAstrom.MeasAstromConfig(sipOrder=sipOrder))
-        newWcs = astrometer.determineWcs(templateSources, templateExposure).getWcs()
-        results = self.register.run(templateSources, newWcs,
+        """Fit the relative astrometry between templateSources and selectSources
+
+        @todo remove this method. It originally fit a new WCS to the template before calling register.run
+        because our TAN-SIP fitter behaved badly for points far from CRPIX, but that's been fixed.
+        It remains because a subtask overrides it.
+        """
+        results = self.register.run(templateSources, templateSources.getWcs(),
                                     templateExposure.getBBox(), selectSources)
         return results