no longer enable Lefty in Autotools builds

no longer enable Lefty in Autotools builds

1a4ad00b59cc1e1e9302fcbd1a205495ee59cab5 made the Autotools build system default
to disabling Lefty as a step towards its deprecation. As three months have
passed, the present commit takes the next step and removes Lefty support from
the portable tarball and stops building it in CI. Users who still want Lefty can
build from source and re-enable it. If we hear from no such users in the next
three months, build system support for Lefty will be removed altogether. If a
further three months go by with no complaints, Lefty will then be deleted from
the repository.