package/systemd: remove libblkid dependency

this dependency is optional, it is used to
allow udev to add information to blockdevices.
Aslong as MOUNT or FSCK are enabled, it will end up enabled
anyway, but this seems more clear and correct.

Signed-off-by: Norbert Lange <nolange79@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>