Revert "Remove remove_data arg for DownloadItem.cancel"

This reverts commit ece85dc903.

It's actually needed in _ask_confirm_question...

qutebrowser/browser/webkit/downloads.py

@@ -481,8 +481,12 @@ class DownloadItem(QObject):
                                            self.stats.percentage(), system)
 
     @pyqtSlot()
-    def cancel(self):
-        """Cancel the download."""
+    def cancel(self, remove_data=True):
+        """Cancel the download.
+
+        Args:
+            remove_data: Whether to remove the downloaded data.
+        """
         log.downloads.debug("cancelled")
         self._read_timer.stop()
         self.cancelled.emit()
@@ -493,7 +497,8 @@ class DownloadItem(QObject):
             self.reply = None
         if self.fileobj is not None:
             self.fileobj.close()
-        self.delete()
+        if remove_data:
+            self.delete()
         self.done = True
         self.finished.emit()
         self.data_changed.emit()