Hi,
The database that you mentioned is it referring to Report Server database or Audit database that Report Server used to generate report? Based on my understanding from your description, it seems you are pointing to Report Server database. Correct me if that's incorrect.
As per my understanding, Report server database is mainly store admin data that related to Report server and shouldn't grow rapidly as compare to Audit database (which capture auth/az audit data). Therefore, there is no document explain on what data need to clean up.
I didn't check what tables reside in Report Server database nor what data it stored. However, if you want to delete any data, I suggest you perform a backup to the database first. In tables, I presume there is timestamp on when the data was being stored. You can make use that as reference if you want to delete data added before 2015. I didn't try to clean data from Report server database before. What I mentioned is for your reference only and suggest to test in lower environment first.
Regards,
Kar Meng