cleanup installicon operation

  - changed errors in warning at the undo step
  - removed unnecessary processEvents call (operations are
    not call in the mainthread since ages)
  - don't remove backups in destructor

Change-Id: Ia2d01400a2eee81024d11b88ccdd9337c9ffbbfa
Reviewed-by: Niels Weber <niels.weber@digia.com>