Keep only seq2seq mode, remove ctc

We only use the seq2seq mode when training DAN, we should remove the ctc mode. See https://gitlab.com/teklia/atr/dan/-/blob/66f0f92db68d23c1312c617c8556169b5423e18c/dan/manager/ocr.py#L55

changed milestone to %DAN-P2: Clean code and remove unused code

added P2 label

assigned to @starride

assigned to @melodie.boillet and unassigned @starride

mentioned in merge request !136 (merged)

closed