There is not a builtin function specifically to undo data removal from brushing. However, one of the following workarounds may work for you:
1. The simplest way to reset your graph would be to click the general 'undo' button for the figure (under 'Edit'). If you backtrack to when you removed the unbrushed points, you can restore the original graph.
2. You could write your own 'brush undo' function to restore the original graph.
When you delete data with the brush tool, the associated 'YData' of the plot is changed to NaN values. Thus, the data cannot be automatically restored without using the general 'undo' button. However, you could work around this by writing a function which resets the 'YData' to the original graph data. For example:
function undoBrush(h, data)
h.YData = data;