Removed jxl feature

[radarhere]

You may have noticed that most of the CI jobs are failing at python-pillow#7848

• Lint is failing because mypy can't find _jxl - https://github.com/python-pillow/Pillow/actions/runs/8121910543/job/22200964297?pr=7848#step:8:17. I've added a stub file here to fix that.
• In many jobs, testing for the "jxl" feature fails - https://github.com/python-pillow/Pillow/actions/runs/8121910544/job/22201592738?pr=7848#step:10:1300. This is because you've written "jxl": ("PIL._jxl", "HAVE_JXL", None), but not defined "HAVE_JXL" in _jxl. I don't think it needs to be a "feature", so I've removed it here.
• macOS Python 3.8 and 3.9 fail to build - https://github.com/python-pillow/Pillow/actions/runs/8121910544/job/22201593322?pr=7848#step:9:256. I imagine you might have thoughts on that though.

[olokelo]

Thank you for the help and explanation.