Stop attempting "legacy editable" installs

[dwpaley]

Legacy-editable mode was removed in pypa/setuptools#4953, this env
variable no longer does anything. We accommodate the new behavior in
cctbx/dxtbx#807 and dials/dials#2906.

[dwpaley]

We can't get rid of legacy editable mode because some builders are pinned to previous versions of dials and dxtbx. Those old versions still expect (in libtbx_refresh.py) to have been installed in legacy editable mode, therefore they add e.g. modules/dials/src to the pythonpath instead of build/dials/lib/python3.x/site-packages. For those builders (at least cctbx and phenix) we will just have to keep setuptools <79 as long as the dials and dxtbx versions remain pinned. For this PR I'll just remove Aaron's workaround in the tests, and scrap the other part for the foreseeable.

[bkpoon]

We can bump up those versions to 3.22. That's the version that we're shipping with Phenix right now since it's the last version to support Python 3.9. The pinned versions in bootstrap.py are because those builders only need things to compile.