Remove/replace LMUCellODE and LMUCellGating

[arvoelke]

We have better versions of these, and they are not currently documented or used by any of the examples.

[celiasmith]

one specific example is the gate initializations seem swapped, in the sense that you want to start the initialization with a bias of one and a weight of zero (otherwise it might never get input to learn off of). for the input gate.

This works:

                 forget_input_kernel_initializer=Constant(0),
                 forget_hidden_kernel_initializer=Constant(0),
                 forget_bias_initializer=Constant(1),