encryption: Fix copy from encrypted multipart to single part (#6604)

CopyObject handler forgot to remove multipart encryption flag in metadata
when source is an encrypted multipart object and the target is also encrypted
but single part object.

This PR also simplifies the code to facilitate review.

cmd/xl-v1-metadata.go

@@ -218,6 +218,8 @@ func (m xlMetaV1) ToObjectInfo(bucket, object string) ObjectInfo {
 		ContentEncoding: m.Meta["content-encoding"],
 	}
 
+	objInfo.backendType = BackendErasure
+
 	// Extract etag from metadata.
 	objInfo.ETag = extractETag(m.Meta)