Blender Git Loki

Fix #37110, After deletion of large scene, file still huge.
Objects were not being freed when unlinked from all scenes, due to user count increments on the ParticleSystem->parent pointers. These were referencing the objects themselves, creating a user count of 1
and preventing free. Object pointers should not usually do user counting, except in some cases like scenes and groups (thanks to Brecht for clarifying this).