Use FileName only to build path in tree service

[KirillovDenis]

Currently for some system objects we use the system tree and SystemName as attribute to build path. I think we can use only FileName to build path because we use different trees (system and versions and they shouldn't conflict). Besides nspcc-dev/neofs-node#1431 allows only FileName.

@alexvanin

[alexvanin]

As long as different trees are solving the problem, let's do that.

[KirillovDenis]

I suppose this issue is blocked by #417 (at least it would be convenient to do this after that issue)

[KirillovDenis]

Done in #461