data/aws/route53/variables: Drop unused external_vpc_id and extra_tags

The last consumer of these variables were removed by 4360905
(modulus,steps: enable existing vpc, 2018-05-16,
coreos/tectonic-installer#3231) back when it was
tectonic_external_vpc_id.  Now all the route53 module creates are
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

@@ -74,8 +74,6 @@ module "dns" {
   elb_alias_enabled        = true
   master_count             = "${var.tectonic_master_count}"
   private_zone_id          = "${local.private_zone_id}"
-  external_vpc_id          = "${module.vpc.vpc_id}"
-  extra_tags               = "${var.tectonic_aws_extra_tags}"
   private_endpoints        = "${local.private_endpoints}"
   public_endpoints         = "${local.public_endpoints}"
 }

data/data/aws/route53/variables.tf

@@ -43,11 +43,6 @@ variable "worker_public_ips_enabled" {
   default     = true
 }
 
-variable "extra_tags" {
-  type        = "map"
-  description = "Extra tags to be applied to created resources."
-}
-
 // AWS specific internal zone variables
 
 variable "elb_alias_enabled" {
@@ -60,17 +55,6 @@ EOF
   default = false
 }
 
-variable "external_vpc_id" {
-  type = "string"
-
-  description = <<EOF
-ID of an existing VPC to launch nodes into.
-If unset a new VPC is created.
-
-Example: `vpc-123456`
-EOF
-}
-
 variable "private_endpoints" {
   description = <<EOF
 If set to true, create private-facing ingress resources (ELB, A-records).