fix: remove parentIsObject() check (#12851)

we will allow situations such as

```
a/b/1.txt
a/b
```

and

```
a/b
a/b/1.txt
```

we are going to document that this usecase is
not supported and we will never support it, if
any application does this users have to delete
the top level parent to make sure namespace is
accessible at lower level.

rest of the situations where the prefixes get
created across sets are supported as is.

cmd/storage-interface.go

@@ -71,7 +71,6 @@ type StorageAPI interface {
 	ReadFileStream(ctx context.Context, volume, path string, offset, length int64) (io.ReadCloser, error)
 	RenameFile(ctx context.Context, srcVolume, srcPath, dstVolume, dstPath string) error
 	CheckParts(ctx context.Context, volume string, path string, fi FileInfo) error
-	CheckFile(ctx context.Context, volume string, path string) (err error)
 	Delete(ctx context.Context, volume string, path string, recursive bool) (err error)
 	VerifyFile(ctx context.Context, volume, path string, fi FileInfo) error
 	StatInfoFile(ctx context.Context, volume, path string) (stat StatInfo, err error)