fc_train - 'objective' function doesn't need 'log_df' parameter

'objective' only uses the 'log_df' parameter when it calls 'train', and 'log_df' is optional for 'train' - it is only needed to store the results of the main training run.

i.e. 'objective' can safely call 'train' without providing 'log_df', so it doesn't need 'log_df' as a parameter.