Hello,
As of Collector for ArcGIS (June 2019), "Using the Files app", you can now import .TPK files AND delete them. When you import a .TPK file into the Collector folder within the Files app. The next time you open Collector, the imported .TPK file will be moved to the Basemaps folder and placed inside a sub-folder with a random character string name. To delete a .TPK basemap file, just open the Collector "folder" in the Files app, open the Basemaps folder, and open each folder until you locate the .TPK basemap file you wish to delete. Delete the folder containing the basemap file.
I agree, this was easier in the Collector Classic app b/c you didn't need to leave the app nor hunt through folders looking for which one holds the .TPK file you want to delete.
Hope this helps!