Revert "Allow help pager to clear screen on exit"

This reverts commit 5a942210.

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

lib/hub/commands.rb

@@ -1046,7 +1046,7 @@ help
         write.close
 
         # Don't page if the input is short enough
-        ENV['LESS'] = 'FSR'
+        ENV['LESS'] = 'FSRX'
 
         # Wait until we have input before we start the pager
         Kernel.select [STDIN]