Is it possible to delete an unnecessary object that was created in the editor's catalog?

It is not possible to remove files from the file and folder structure using the editor's tools. All links are generated and stored in index.bin; if you understand its structure, you can manually remove links (spoiler: this is possible). The editor does not work with archived files (maps are archives). It first unpacks them into a temporary folder, Editor/H5MMods/MapName/, then modifies the files there during map editing. Upon closing the map or selecting Resource => Pack MOD, it archives them back. The [MOD: some path] bar in the application title indicates that the editor is currently working in a folder with unpacked data. To exit this folder, press Resource => Close MOD (on the toolbar); the editor will then work in the data directory without prior unpacking/packing. Keep in mind that each time you open a map, it deletes the previous files from Editor/H5MMods/MapName/, so working with a map as an archive for a future mod is not the correct approach.