Why do you have to uninstall the card reader? You can upgrade to Windows 8 without uninstalling the card reader also.
If you need to uninstall the card reader, go to start->type devmgmt.msc->press enter. Expand IDE ATA/ATAPI Controller->look for Realtek PCIE Card Reader. Double click on it->go to Driver tab. Click Uninstall (select Delete the software for this device to uninstall the device completely) to uninstall the device.
Note: Multiple drivers may be present (like SD controller, xD controller, MemoryStick controller). Uninstall them individually.