Remove unneeded ignore_tabs_are_windows argument

This was originally introduced in 57adcea587
for #2162. However, support for Qt < 5.7.1 was dropped long ago in
a86170f45d, so this can be removed.

qutebrowser/mainwindow/tabbedbrowser.py

@@ -553,8 +553,7 @@ class TabbedBrowser(QWidget):
             self, url: QUrl = None,
             background: bool = None,
             related: bool = True,
-            idx: int = None, *,
-            ignore_tabs_are_windows: bool = False
+            idx: int = None,
     ) -> browsertab.AbstractTab:
         """Open a new tab with a given URL.
 
@@ -573,8 +572,6 @@ class TabbedBrowser(QWidget):
                          - Explicitly opened tabs are at the very right
                            (related=False)
             idx: The index where the new tab should be opened.
-            ignore_tabs_are_windows: If given, never open a new window, even
-                                     with tabs.tabs_are_windows set.
 
         Return:
             The opened WebView instance.
@@ -587,8 +584,7 @@ class TabbedBrowser(QWidget):
 
         prev_focus = QApplication.focusWidget()
 
-        if (config.val.tabs.tabs_are_windows and self.widget.count() > 0 and
-                not ignore_tabs_are_windows):
+        if config.val.tabs.tabs_are_windows and self.widget.count() > 0:
             window = mainwindow.MainWindow(private=self.is_private)
             window.show()
             tabbed_browser = objreg.get('tabbed-browser', scope='window',