Remove hard tanh activation function header.
Remove hard tanh activation function header.

ranjan1990/mlpack,darcyliu/mlpack,palashahuja/mlpack,ajjl/mlpack,darcyliu/mlpack,palashahuja/mlpack,darcyliu/mlpack,ajjl/mlpack,palashahuja/mlpack,ranjan1990/mlpack,ranjan1990/mlpack,ajjl/mlpack