[Bf-python] Remove a modal handler from the window manager

[Dan Eicher]

Somewhere in VIEW3D_OT_display_measurements there needs to be a check
for canceling the drawing operation which then calls:

...region.callback_remove(self._handle)

-- perhaps in modal() which calls a dedicated function to loop over
all the windows/regions and clean this stuff up.

Also doesn't seem to be a call to remove the handler either but that
is point #1 above...

Dan