remove dotty, lefty, lneato

Lefty, and its dependent tools Dotty and Lneato, have been suspected unused for a long time. They were scheduled for removal, but we took a cautious and very slow approach to this given how pervasive Graphviz is and the limited visibility we have into its usage:

1. 2021-09-25 make Lefty default to disabled in the build system¹
2. 2022-01-16 disable Lefty in published graphviz.org packages²
3. 2022-03-28 remove build system support³

This completes the final step of removing the code from the repository. It is still retained in the history, should anyone need access to it or need to build a past version of Graphviz with Lefty.

So far, we have only heard two notable reactions from the downstream world:

1. Lefty disabling was not quite complete on the first attempt.⁴
2. There has been an offer of assistance to maintain Lefty.⁵ No patches have yet emerged, but if and when they do we could feasibly reverse direction on this and reinstate Lefty and friends.

Gitlab: closes #219 (closed), closes #552 (closed), related to #1836 (closed), #1854 (closed)

¹ 1a4ad00b ² 04a78cc9 ³ 6faa3401 ⁴ #2153 (closed) ⁵ https://forum.graphviz.org/t/lefty-not-a-graphviz-application/835/6