Remove never changed training parameters

A lot of parameters (saved in params file during training) are never changed (and maybe not even used?). We should remove them from the configuration. Here is a (non-exhaustive) list:

• input_channels: we always convert to RGB
• use_1d_pe: always True
• use_2d_pe: always True
• use_lstm: always False, maybe not even used
• max_training_time: used for Jean Zay, I never used it, WDYT @starride?
• interval_save_weights
• focus_metric and expected_metric_value: we always use the CER on validation set so we can fix these

Removing these parameters should not change the training nor the prediction.