Do not test for a bug in older GnuPG versions

* tests/json/t-keylist-secret.out.json

--
Since at least 2.2.19 GnuPG no longer emits secret = true
for keys where it does not have a secret. To avoid
failures with older versions we remove the check altogether.

GnuPG-Bug-Id: T4820

tests/json/t-keylist-secret.out.json

@@ -91,7 +91,6 @@
             "can_sign":     true,
             "can_certify":  true,
             "can_authenticate":     true,
-            "secret":       true,
             "is_qualified": false,
             "protocol":     "OpenPGP",
             "fingerprint":  "D695676BDCEDCC2CDD6152BCFE180B1DA9E3B0B2",