`DynetParams` object doesn't work any more.

In the document [here](http://dynet.readthedocs.io/en/latest/python_ref.html), the sample code below doesn't work anymore. I don't know how this mechanism works before adding `dynet_config` since it will init twice which is not allowed (https://github.com/clab/dynet/blob/master/dynet/init.cc#L220).

```
# Declare a DynetParams object
dyparams = dy.DynetParams()
# Fetch the command line arguments (optional)
dyparams.from_args()
# Set some parameters manualy (see the command line arguments documentation)
dyparams.set_mem(2048)
dyparams.set_random_seed(666)
# Initialize with the given parameters
dyparams.init() # or init_from_params(dyparams)
```

If no one uses this anymore, can we remove this both from code and document?