sd-device: reduce the number of implementations of device_read_db() we keep around

sd-device: reduce the number of implementations of device_read_db() we keep around

We had two very similar functions: device_read_db_aux and device_read_db,
and a number of wrappers for them:

device_read_db_aux
  ← device_read_db (in sd-device.c)
    ← all functions in sd-device.c, including sd_device_is_initialized

  ← device_read_db_force
     ← event_execute_rules_on_remove (in udev-event.c)

device_read_db (in device-private.c)
  ← functions in device_private.c (but not device_read_db_force):
    device_get_devnode_{mode,uid,gid}
    device_get_devlink_priority
    device_get_watch_handle
    device_clone_with_db
    ← called from udevadm, udev-{node,event,watch}.c

Before 7141e4f62c3f220872df3114c42d9e4b9525e43e (sd-device: don't retry loading
uevent/db files more than once), the two implementations were the same. In that
commit, device_read_db_aux was changed. Those changes were reverted in the parent
commit, so the two implementations are now again the same except for superficial
differences. This commit removes device_read_db (in sd-device.c), and renames
device_read_db_aux to device_read_db_internal and makes everyone use this one
implementation. There should be no functional change.