There were some Hive tables that we used for session length testing that are not necessary any more.
Those tables had associated data in HDFS, as well as Superset charts and dashboards.
I removed all of them.
Also, the production intermediate table wmf.session_length_daily had some incorrect data up to March 18th (included),
because of T277515 and https://gerrit.wikimedia.org/r/c/mediawiki/extensions/WikimediaEvents/+/672722.
I deleted the data from early March 2021 up to the 18th, included.