Reuse notary request event type subscription filters where possible

[AnnaShaleva]

Is your feature request related to a problem? Please describe.

Since https://github.com/nspcc-dev/neo-go/pull/3236 (which is going to be a part of 0.105.0) there's an ability to subscribe only for added or removed notary requests WS notifications. Likely we don't need removed notary requests at the whole NeoFS system, and what we need to use is added filter for notary request subscriptions.