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/api-errors.go

@@ -231,7 +231,6 @@ const (
 	// MinIO extended errors.
 	ErrReadQuorum
 	ErrWriteQuorum
-	ErrParentIsObject
 	ErrStorageFull
 	ErrRequestBodyParse
 	ErrObjectExistsAsDirectory
@@ -1122,11 +1121,6 @@ var errorCodes = errorCodeMap{
 		Description:    "Storage backend has reached its minimum free disk threshold. Please delete a few objects to proceed.",
 		HTTPStatusCode: http.StatusInsufficientStorage,
 	},
-	ErrParentIsObject: {
-		Code:           "XMinioParentIsObject",
-		Description:    "Object-prefix is already an object, please choose a different object-prefix name.",
-		HTTPStatusCode: http.StatusBadRequest,
-	},
 	ErrRequestBodyParse: {
 		Code:           "XMinioRequestBodyParse",
 		Description:    "The request body failed to parse.",
@@ -1899,8 +1893,6 @@ func toAPIErrorCode(ctx context.Context, err error) (apiErr APIErrorCode) {
 		apiErr = ErrObjectExistsAsDirectory
 	case PrefixAccessDenied:
 		apiErr = ErrAccessDenied
-	case ParentIsObject:
-		apiErr = ErrParentIsObject
 	case BucketNameInvalid:
 		apiErr = ErrInvalidBucketName
 	case BucketNotFound: