Delete Irrelevant open command for unix

[tokusumi]

open command works to handle browser for only MacOS.
For Unix, it cause the followings error:

$ cargo test
...
running 1 test
Couldn't get a file descriptor referring to the console
test tests::test_open_default ... FAILED

failures:

---- tests::test_open_default stdout ----
thread 'tests::test_open_default' panicked at 'failed to open browser: Custom { kind: Other, error: "return code 1" }', tests/common.rs:61:36

I found this error in WSL2 environments (no xdg-open, gvfs-open, gnome-open but x-www-browser is only available).
As a discussed here, open command in Unix is link to openvt command, which is not to open browser.
This cause panic and the process finish before calling x-www-browser.

This PR fix above problem and works fine by calling x-www-browser.

[amodm]

Thanks for fixing this, @tokusumi. LGTM. This will go out in the upcoming release (planned sometime this week).

[amodm]

Documenting for future reference. The open command was originally added to support Haiku OS, so removal would break things on it. I've made appropriate changes in 696dd54 to make sure that open gets called only for Haiku, and not on other Unixes.