removing preferences that were removed from firefox

Fixes #2117

Author: lukeis

javascript/firefox-driver/webdriver.json

@@ -40,15 +40,6 @@
     "security.csp.enable": false,
     "security.fileuri.origin_policy": 3,
     "security.fileuri.strict_origin_policy": false,
-    "security.warn_entering_secure": false,
-    "security.warn_entering_secure.show_once": false,
-    "security.warn_entering_weak": false,
-    "security.warn_entering_weak.show_once": false,
-    "security.warn_leaving_secure": false,
-    "security.warn_leaving_secure.show_once": false,
-    "security.warn_submit_insecure": false,
-    "security.warn_viewing_mixed": false,
-    "security.warn_viewing_mixed.show_once": false,
     "signon.rememberSignons": false,
     "toolkit.networkmanager.disable": true,
     "toolkit.telemetry.prompted": 2,