Re-re-remove deprecated Qt globals.

These were missed in 1d15bf4, and caused import of `backend_qt5` to fail
with `cannot import name 'SUPER' from 'matplotlib.backends.backend_qt'`.

Author: anntzer

lib/matplotlib/backends/backend_qt5.py

@@ -1,7 +1,5 @@
 from .backend_qt import (
     backend_version, SPECIAL_KEYS,
-    # These are deprecated.
-    SUPER, ALT, CTRL, SHIFT, MODIFIER_KEYS as _MODIFIER_KEYS,
     # Public API
     cursord, _create_qApp, _BackendQT, TimerQT, MainWindow, FigureCanvasQT,
     FigureManagerQT, ToolbarQt, NavigationToolbar2QT, SubplotToolQt,