Remove transcritpions sources count filter

The filter with_transcription_sources_count will not be used anymore after frontend!667 (merged) is merged.
It can easily be removed from ListElement endpoints

added 1 deleted label

changed the description

mentioned in merge request !904 (merged)

closed

mentioned in commit 245984bf

reopened

The merge request is OK, but we can't merge it yet, as the frontend still needs that for 0.14.0 !

It got merged though

Wasn't frontend!667 (merged) alone enough?

mentioned in merge request !905 (merged)

closed

mentioned in commit 077eff97