[#1130] fstree: Remove useless Stat() call

```
goos: linux
goarch: amd64
pkg: git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/blobstor
cpu: 11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz
                                           │     old     │                new                 │
                                           │   sec/op    │   sec/op     vs base               │
SubstorageReadPerf/fstree_nosync-seq100-8    2.689µ ± 2%   2.428µ ± 4%  -9.72% (p=0.000 n=10)
SubstorageReadPerf/fstree_nosync-rand100-8   2.727µ ± 1%   2.497µ ± 2%  -8.42% (p=0.000 n=10)
geomean                                      2.708µ        2.462µ       -9.07%
```

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

pkg/local_object_storage/blobstor/fstree/fstree.go

@@ -358,10 +358,6 @@ func (t *FSTree) Get(ctx context.Context, prm common.GetPrm) (common.GetRes, err
 
 	p := t.treePath(prm.Address)
 
-	if _, err := os.Stat(p); os.IsNotExist(err) {
-		return common.GetRes{}, logicerr.Wrap(new(apistatus.ObjectNotFound))
-	}
-
 	var data []byte
 	var err error
 	{