Disable smooth scrolling.

@philc.  Even with #1324 there are issues related to smooth scrolling when zoomed.  I suggest we should disable it entirely for now.

Issues:
- `gg` might not take us to the top of the screen.
- We may not be able to scroll out of sub-elements (not sure about this).
- The scroll speed is affected by the zoom level.

These can all be solved, but we need time to get the solution right.