test_detect_attic_repo: don't test mount

since mount is not always available and if it works for all the other
commands, then it is likely it works for mount as well.

(cherry picked from commit a6be34f8f7e803dcc0ec8934c2b70d90e5ea03a7)

src/borg/testsuite/archiver.py

@@ -2745,7 +2745,6 @@ id: 2 / e29442 3506da 4e1ea7 / 25f62a 5a3d41 - 02
             ['delete', path],
             ['prune', path],
             ['info', path + '::test'],
-            ['mount', path, self.tmpdir],
             ['key', 'export', path, 'exported'],
             ['key', 'import', path, 'import'],
             ['change-passphrase', path],