Make BUILDOUT-NEXT test suite use same setuptools version as master again

In 51740961 we "started testing new version of setuptools" and `BUILDOUT-NEXT` test suite was set to use this `software/buildout-testing/software-next.cfg`.

In nexedi/slapos!425 we started to depend on very recent setuptools and updated to 40.4.3 .

This `software/buildout-testing/software-next.cfg` kept using this old setuptools and `BUILDOUT-NEXT` test suite failed to build in a loop.

The test suite was already changed to use `software/buildout-testing/software.cfg` (ie. it's same as `BUILDOUT` but testing slapos.buildout's  `next` branch instead of `master` ), so I think this profile is not needed currently.

/reviewed-on nexedi/slapos!488

software/buildout-testing/software-next.cfg

-[buildout]
-extends = software.cfg
-
-[versions]
-setuptools = 36.6.0