remove build system support for 'lefty'

remove build system support for 'lefty'

04a78cc9ae01b2a7e5d5435146cd503c45af90b9 disabled Lefty in the Autotools build
system. The only fall out from this seems to have been Gentoo discovering
mistakes in the Graphviz Autotools files.¹ With mild evidence that there is no
one downstream relying on Lefty, we take the next step in its removal. If
another three months pass with no issues, we can remove Lefty from the Graphviz
tree altogether.

CC: David Seifert <soap@gentoo.org>
Gitlab: related to #1836, #1854

¹ https://gitlab.com/graphviz/graphviz/-/merge_requests/2520