Fix two errors in {Statistics,Achievements}Asset

REVIEW: 103452
1. The deletion of Achievements/Statistics when calling unload makes
the creator crash. Solution: Don't unload. Most assets don't do that
either.
2. The automatic loading caused an endless recursion, as the gdl parser
checks whether the object already exists in the object tree, which
causes load() to be called again. Solution: mark as loaded earlier.