Remove inactive username_password_not_available test

This is cleanup related to #1644, which further complicated the
attributes on the `#[ignore]`d `username_password_not_available`
test with an empty that was build (but never run) whenever the
`expectrl` tests of `gix-prompt` were not built. Instead of keeping
its attributes more complicated and maintaining an expectation that
they continue to be updated along with the `target_os` support for
`expectrl`'s `ptyprocess` dependency, this removes that empty test.

The purpose of the test case that this removes is unclear, and it
may not have value anymore. It is unclear in part because actual
interaction to provide a username and password is likely to work on
some systems for which the related tests that use `expectrl` are
not being built. But this empty test case might be read to say
otherwise.

See also the related discussion at:
#1644 (review)

Author: EliahKagan

gix-prompt/tests/prompt.rs

@@ -63,9 +63,4 @@ mod ask {
         p.expect("\" password with space \"").unwrap();
         p.expect(expectrl::Eof).unwrap();
     }
-
-    #[test]
-    #[cfg(not(any(target_os = "linux", target_os = "freebsd", target_os = "macos")))]
-    #[ignore]
-    fn username_password_not_available() {}
 }