Fixed previewer.py::render_card() being called twice in row

Related to: https://github.com/ankitects/anki/pull 529
Ensuring "setNote" is called only once during changing of note type

After debugging the code, every time the cards changes, there are
two stacktraces generated and triggered when note cards change:

calling 1590591251.4621403
   File "qt/runanki", line 4, in <module>
  File "F:\anki\qt\aqt\__init__.py", line 365, in run
    _run()
  File "F:\anki\qt\aqt\__init__.py", line 515, in _run
    app.exec()
  File "F:\anki\qt\aqt\webview.py", line 467, in handler
    cb(val)
  File "F:\anki\qt\aqt\editor.py", line 477, in <lambda>
    self.web.evalWithCallback("saveNow(%d)" % keepFocus, lambda res: callback())
  File "F:\anki\qt\aqt\browser.py", line 858, in <lambda>
    self.editor.saveNow(lambda: self._onRowChanged(current, previous))
  File "F:\anki\qt\aqt\browser.py", line 880, in _onRowChanged
    self._renderPreview(True)
  File "F:\anki\qt\aqt\browser.py", line 1576, in _renderPreview
    self._previewer.render_card(cardChanged)
  File "F:\anki\qt\aqt\previewer.py", line 136, in render_card
    print('calling', time.time(), "\n", "".join( traceback.format_stack() ) )

calling 1590591251.4766161
   File "qt/runanki", line 4, in <module>
  File "F:\anki\qt\aqt\__init__.py", line 365, in run
    _run()
  File "F:\anki\qt\aqt\__init__.py", line 515, in _run
    app.exec()
  File "F:\anki\qt\aqt\webview.py", line 467, in handler
    cb(val)
  File "F:\anki\qt\aqt\editor.py", line 453, in oncallback
    gui_hooks.editor_did_load_note(self)
  File "F:\anki\qt\aqt\gui_hooks.py", line 1168, in __call__
    hook(editor)
  File "F:\anki\qt\aqt\browser.py", line 887, in onLoadNote
    self.refreshCurrentCard(editor.note)
  File "F:\anki\qt\aqt\browser.py", line 884, in refreshCurrentCard
    self._renderPreview(False)
  File "F:\anki\qt\aqt\browser.py", line 1576, in _renderPreview
    self._previewer.render_card(cardChanged)
  File "F:\anki\qt\aqt\previewer.py", line 136, in render_card
    print('calling', time.time(), "\n", "".join( traceback.format_stack() ) )

qt/aqt/browser.py

@@ -880,7 +880,6 @@ QTableView {{ gridline-color: {grid} }}
 
     def refreshCurrentCard(self, note: Note) -> None:
         self.model.refreshNote(note)
-        self._renderPreview(False)
 
     def onLoadNote(self, editor):
         self.refreshCurrentCard(editor.note)