Remove dead code.

This line doesn’t appear to do anything useful, since the parent node is
now saved in the closure.  Previously, this line checked to see if
target.parentNode was null, since this would cause a crash when
computing the relative position.

d3.js

@@ -1131,7 +1131,6 @@ d3 = function() {
           type: "dragstart"
         });
         function moved() {
-          if (!parent) return ended();
           var p = position(parent, eventId), dx = p[0] - origin_[0], dy = p[1] - origin_[1];
           dragged |= dx | dy;
           origin_ = p;

src/behavior/drag.js

@@ -49,8 +49,6 @@ d3.behavior.drag = function() {
       event_({type: "dragstart"});
 
       function moved() {
-        if (!parent) return ended(); // target removed from DOM
-
         var p = position(parent, eventId),
             dx = p[0] - origin_[0],
             dy = p[1] - origin_[1];