Fixed APN names

Author: toyowata

mbed_app.json

@@ -21,10 +21,10 @@
             "nsapi.default-wifi-security"               : "WPA_WPA2",
             "nsapi.default-wifi-ssid"                   : "\"SSID\"",
             "nsapi.default-wifi-password"               : "\"PASSWORD\"",
-            "nsapi.default-cellular-apn"                : "\"soracom.io\"",
+            "nsapi.default-cellular-apn"                : "\"APN\"",
             "nsapi.default-cellular-sim-pin"            : null,
-            "nsapi.default-cellular-username"           : "\"sora\"",
-            "nsapi.default-cellular-password"           : "\"sora\""
+            "nsapi.default-cellular-username"           : null,
+            "nsapi.default-cellular-password"           : null
         },
         "Freescale": {
             "lwip.mem-size"                             : 12500