data/data/aws/vpc: Drop 'current' from aws_region

It's the default.  The old parameter was deprecated before
hashicorp/terraform-provider-aws@1cc81c92
(docs/data-source/aws_region: Remove now deprecated current argument,
2018-02-09, v1.9.0), and we're pinning 1.39.0 since ac5aeed
(data/aws: bump aws provider, 2018-11-01, openshift#594).  Changing this
avoids:

  $ openshift-install --log-level=debug create cluster
  ...
  - Downloading plugin for provider "aws" (1.39.0)...
  ...
  Warning: module.vpc.data.aws_region.current: "current": [DEPRECATED] Defaults to current provider region if no other filtering is enabled
  ...

data/data/aws/vpc/common.tf

@@ -1,8 +1,6 @@
 # Canonical internal state definitions for this module.
 # read only: only locals and data source definitions allowed. No resources or module blocks in this file
-data "aws_region" "current" {
-  current = true
-}
+data "aws_region" "current" {}
 
 // Fetch a list of available AZs
 data "aws_availability_zones" "azs" {}