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/object-api-errors.go

@@ -22,7 +22,6 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"path"
 )
 
 // Converts underlying storage error. Convenience function written to
@@ -64,15 +63,6 @@ func toObjectErr(err error, params ...string) error {
 			apiErr.Object = decodeDirObject(params[1])
 		}
 		return apiErr
-	case errFileParentIsFile.Error():
-		apiErr := ParentIsObject{}
-		if len(params) >= 1 {
-			apiErr.Bucket = params[0]
-		}
-		if len(params) >= 2 {
-			apiErr.Object = decodeDirObject(params[1])
-		}
-		return apiErr
 	case errIsNotRegular.Error():
 		apiErr := ObjectExistsAsDirectory{}
 		if len(params) >= 1 {
@@ -322,13 +312,6 @@ func (e PrefixAccessDenied) Error() string {
 	return "Prefix access is denied: " + e.Bucket + SlashSeparator + e.Object
 }
 
-// ParentIsObject object access is denied.
-type ParentIsObject GenericError
-
-func (e ParentIsObject) Error() string {
-	return "Parent is object " + e.Bucket + SlashSeparator + path.Dir(e.Object)
-}
-
 // BucketExists bucket exists.
 type BucketExists GenericError