[#1667] shard: Drop shard pool

After adding an ops limiter, shard's `put` pool is redundant.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

cmd/frostfs-adm/internal/modules/storagecfg/config.go

@@ -40,8 +40,6 @@ morph:
     - address: wss://{{.}}/ws{{end}}
 {{if not .Relay }}
 storage:
-  shard_pool_size: 15  # size of per-shard worker pools used for PUT operations
-
   shard:
     default: # section with the default shard parameters
       metabase: