Revert "Allow help pager to clear screen on exit"

This reverts commit 5a94221027e8c83a08ee15ff03f21ab74e288f1d.

Removing "X" from `less` configuration did help to clear the screen on
exit, but also led to paginated output being swallowed up when the
output was less than one page (i.e. one line). I have no idea what
actually happens, but it's important to bring back full output.

I couldn't find a way to have paginated output cleared on exit at the
same time as having single-page output normally.

Fixes #520, reopens #368