Remove deprecated toolbar attributes

As far as I can tell these were leftover attributes from
a very old version of the matplotlib NavigationToolbarQT
class that have now been deprecated in matplotlib 3.1 and
set as readonly. A search back through matplotlib 1.5
suggests that they were not used and 1.5 is the minimum we
are required to support.

qt/applications/workbench/workbench/plotting/toolbar.py

@@ -76,7 +76,6 @@ class WorkbenchNavigationToolbar(NavigationToolbar2QT):
                 if tooltip_text is not None:
                     a.setToolTip(tooltip_text)
 
-        self.buttons = {}
         # Add the x,y location widget at the right side of the toolbar
         # The stretch factor is 1 which means any resizing of the toolbar
         # will resize this label instead of the buttons.
@@ -88,9 +87,6 @@ class WorkbenchNavigationToolbar(NavigationToolbar2QT):
             labelAction = self.addWidget(self.locLabel)
             labelAction.setVisible(True)
 
-        # reference holder for subplots_adjust window
-        self.adj_window = None
-
         # Adjust icon size or they are too small in PyQt5 by default
         dpi_ratio = QtWidgets.QApplication.instance().desktop().physicalDpiX() / 100
         self.setIconSize(QtCore.QSize(24 * dpi_ratio, 24 * dpi_ratio))