Remove claim that Parsl is tested on macOS

[benclifford]

Type of change

• Update to human readable text: Documentation/error messages/comments

[khk-globus]

Being pedantic, Parsl has been tested on macOS, but until someone manually does it again, the most recent release may not have been, right? But we don't have a CI setup for it, which is what this sentence is attempting to get at, yes?

[benclifford]

I saw enough trouble eg with Outreachy students trying to do things on os x that I'm not even particularly willing to say it has been tested on OS X manually... some people seem to run some pieces of Parsl on OS X, but in the last year I've also encountered plenty of badness.