resource/aws_rds_cluster: Remove extraneous conditional
resource/aws_rds_cluster: Remove extraneous conditional

Co-Authored-By: bflad <2d9d7b80898d9ce2184cfa69cb628eece272cafc@gmail.com>
Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws