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-errors.go

@@ -88,9 +88,6 @@ var errFileAccessDenied = StorageErr("file access denied")
 // errFileCorrupt - file has an unexpected size, or is not readable
 var errFileCorrupt = StorageErr("file is corrupted")
 
-// errFileParentIsFile - cannot have overlapping objects, parent is already a file.
-var errFileParentIsFile = StorageErr("parent is a file")
-
 // errBitrotHashAlgoInvalid - the algo for bit-rot hash
 // verification is empty or invalid.
 var errBitrotHashAlgoInvalid = StorageErr("bit-rot hash algorithm is invalid")