narrow: remove hack to write narrowspec to shared .hg directory

AFAIK, we no longer need it since the narrowspec file was move to the
store directory in 576eef1ab43d, "narrow: move .hg/narrowspec to
.hg/store/narrowspec."

mercurial/localrepo.py

@@ -841,11 +841,7 b' class localrepository(object):'
         return self._narrowmatch
 
     def setnarrowpats(self, newincludes, newexcludes):
-        target = self
-        if self.shared():
-            from . import hg
-            target = hg.sharedreposource(self)
-        narrowspec.save(target, newincludes, newexcludes)
+        narrowspec.save(self, newincludes, newexcludes)
         self.invalidate(clearfilecache=True)
 
     def __getitem__(self, changeid):