data/aws/vpc: Drop unused depends_on variable

This variable was added in a2232f3 (platform/aws: create bootstrap
step, 2018-02-14, coreos/tectonic-installer#2946), to force VPC
creation after the S3 bucket.  But that dependency was removed in
b2e0bcf (Don't upload /generated to S3 - only use Ignition,
2018-04-13, coreos/tectonic-installer#3183), so we no longer need the
variable.

data/data/aws/vpc/variables.tf

@@ -51,8 +51,3 @@ variable "public_master_endpoints" {
   description = "If set to true, public-facing ingress resources are created."
   default     = true
 }
-
-variable "depends_on" {
-  default = []
-  type    = "list"
-}