Remove jing_tools expression (now contained in jing expression itself)

The jing expression now creates its own "jing" wrapper script, so there
is no need for jing_tools anymore.

jing hasn't been updated in years, so I assume (or hope) that not many
(if any) have jing_tools in their configuration.nix. If you do, just
change it to jing and it should behave the same.