data/aws/route53/variables: Drop unused extra_tags

The last consumer of these variables was removed by 4360905
(modulus,steps: enable existing vpc, 2018-05-16,
coreos/tectonic-installer#3231).  Now the route53 module only creates
aws_route53_record resources, and they don't support tags [1].

[1]: https://www.terraform.io/docs/providers/aws/r/route53_record.html

data/data/aws/main.tf

@@ -63,7 +63,6 @@ module "dns" {
   cluster_name             = "${var.cluster_name}"
   master_count             = "${var.master_count}"
   private_zone_id          = "${local.private_zone_id}"
-  extra_tags               = "${var.aws_extra_tags}"
 }
 
 module "vpc" {

data/data/aws/route53/variables.tf

@@ -25,11 +25,6 @@ variable "worker_ip_addresses" {
   default     = []
 }
 
-variable "extra_tags" {
-  type        = "map"
-  description = "Extra tags to be applied to created resources."
-}
-
 // AWS specific internal zone variables
 
 variable "private_zone_id" {