Remove keep_only_ner_tokens and edit_wer_from_string from metrics

Remove keep_only_ner_tokens and edit_wer_from_string as these functions are never used. See https://gitlab.com/teklia/atr/dan/-/blob/2adb6ba8e9f7698b3f7968292a87c6dde920f42b/dan/manager/metrics.py#L211 and https://gitlab.com/teklia/atr/dan/-/blob/2adb6ba8e9f7698b3f7968292a87c6dde920f42b/dan/manager/metrics.py#L181