cli: Don't give character value 3 (CTRL-C) any special treatment. [#57] (#64)

This was originally here because of some misconfigured TTY settings,
and can cause problems when running on embedded devices.

Since the misconfigured TTY settings have been addressed, this check
is no longer needed.

src/cli/cli_serial.cpp

@@ -102,10 +102,6 @@ void Serial::ReceiveTask(void)
 
             break;
 
-        case 3: // CTRL-C
-            exit(1);
-            break;
-
         case '\b':
         case 127:
             otPlatSerialSend(sEraseString, sizeof(sEraseString));