ssh: don't install a client by default (#4016)

Make use of the recently added nullable `mkPackageOption` feature
to disable installing an SSH client by default: most people should use
the client provided by their system.

tests/modules/programs/ssh/forwards-dynamic-valid-bind-no-asserts.nix

@@ -27,8 +27,6 @@ with lib;
     home.file.result.text = builtins.toJSON
       (map (a: a.message) (filter (a: !a.assertion) config.assertions));
 
-    test.stubs.openssh = { };
-
     nmt.script = ''
       assertFileExists home-files/.ssh/config
       assertFileContent \