Remove change to mbed-os.lib; Set default APN back to 'APN'

Author: Chris Trowbridge

mbed_app.json

@@ -21,7 +21,7 @@
             "nsapi.default-wifi-security"               : "WPA_WPA2",
             "nsapi.default-wifi-ssid"                   : "\"SSID\"",
             "nsapi.default-wifi-password"               : "\"PASSWORD\"",
-            "nsapi.default-cellular-apn"                : "\"phone\"",
+            "nsapi.default-cellular-apn"                : "\"APN\"",
             "nsapi.default-cellular-sim-pin"            : null,
             "nsapi.default-cellular-username"           : null,
             "nsapi.default-cellular-password"           : null