This reverts commit f0b4aa5d59.
This commit was making files to get bigger and bigger every time they
were saved and re-opened.
In the orphaned data in the outliner new collections would show up
there, even after continuously purging it. This would lead to a massive
file which get also very slow.
This problem will fix itself after a few re-open/re-saves of the files.
For anyone also experimenting this you can fix this faster by purging
the unused data multiple times in the file.
Example of file from the Project Heist (rev. 1014):
heist/pro/shots/010_opening/010_0050/010_0050.anim.blend
31d80ddeaa