[#1161] node: Remove notification functionality

It is unused and will be reworked in future.

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

cmd/frostfs-node/config.go

@@ -474,7 +474,6 @@ type cfg struct {
 	cfgNetmap         cfgNetmap
 	cfgControlService cfgControlService
 	cfgObject         cfgObject
-	cfgNotifications  cfgNotifications
 }
 
 // ReadCurrentNetMap reads network map which has been cached at the
@@ -633,12 +632,6 @@ type cfgObject struct {
 	skipSessionTokenIssuerVerification bool
 }
 
-type cfgNotifications struct {
-	enabled      bool
-	nw           notificationWriter
-	defaultTopic string
-}
-
 type cfgLocalStorage struct {
 	localStorage *engine.StorageEngine
 }