Making real need to remove proxies

Otherwise some invalid pointers will be left
which could be harmless if real object stays
local, but crashes when linking them to
another files.

Was discovered here in the studio during
Project Pampa, and the steps to reproduce are:

- Create lib.blend, put armature and cube to it.
  Create a group with them.
- Create scene.blend and link group from lib.blend.
- Make a proxy from armature.
- Make group real.
- Add real objects to a group.
- Create comp.blend and link group from scene.blend.
  This step will creah.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60898 954f8c5b-7b00-dc11-b283-0030488c597c