Re #8239. Remove unused undo stack code.

This is inherited code from qtiPlot that was disabled on our
side a long time ago. It provided undo functionality on matrices.
Re #8329. Remove unused undo/redo action.

This hooked up undo and redo actions to menu items, but these menu
items are long-since removed from MantidPlot. The functionality had
also gone from Matrices, but is still there via the keyboard shortcuts
in Notes (if anyone would ever use it anyway).
Merge remote-tracking branch 'origin/feature/8239_unused_undo_redo_code_applicationwindow'