core: unset sysfs path after transition to dead state (#6174)

core: unset sysfs path after transition to dead state (#6174)

Device is gone and most likely it will get garbage collected. However in
cases when it doesn't get gc'ed (because it is referenced by some
other unit, e.g. mount from fstab) we need to unset sysfs. This is
because when device appears next time, possibly, with different sysfs
path we need to update the sysfs path. Current code could end up caching
stale sysfs path forever.

In reality this is not a problem for normal disks (unless you swap them
during system runtime). However this issue causes failures to mount
filesystems on LVM where sysfs path depends on activation
order (i.e. logical volumes from volume group that is activated first
get assigned lower dm-X numbers and corresponding syspaths).

Fixes #6126.