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).

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60836 954f8c5b-7b00-dc11-b283-0030488c597c