[scAPP] model: remove `experiment_type` from singecell object

[biomystery]

Otherwise, it could easiliy happen that the info here is not consistent with metaData's info.

SOmetime, ppl entered wrong experimental type and need correct it in metaData app. So we should not store it in two different places.

[biomystery]

eg:

spreissl 9:14 AM
@frank @Yuxin you could you please remove the processed files for MM_405-408 and MM_425,MM_426 from LIMS they were processed as scRNA-seq, I have now updated the metadata to snRNA-seq