Remove duplicate case statement

This line was never being hit due to the duplicate one above (#L166).
Peeking at the server code shows that 403 is only given in response to
registration attempts, where it does in fact indicate an invalid code.

// FREEBIE

js/libtextsecure.js

@@ -36354,7 +36354,6 @@ var TextSecureServer = (function() {
                     message = "Number already registered.";
                     break;
                 case 401:
-                case 403:
                     message = "Invalid authentication, most likely someone re-registered and invalidated our registration.";
                     break;
                 case 404:

libtextsecure/api.js

@@ -171,7 +171,6 @@ var TextSecureServer = (function() {
                     message = "Number already registered.";
                     break;
                 case 401:
-                case 403:
                     message = "Invalid authentication, most likely someone re-registered and invalidated our registration.";
                     break;
                 case 404: