Remove unactionable FIXME in `InstallRequirement`
Remove unactionable FIXME in `InstallRequirement`

`setup.py develop` doesn't accept `--install-headers`, so there's no
need for this comment.

rouge8/pip,pypa/pip,xavfernandez/pip,sbidoul/pip,rouge8/pip,sbidoul/pip,pypa/pip,xavfernandez/pip,pradyunsg/pip,rouge8/pip,xavfernandez/pip,pfmoore/pip,pradyunsg/pip,pfmoore/pip