data/aws/vpc/master-elb: Drop idle_timeout from aws_lb resources

It is not valid for network load balancers [1].  We've been setting
this in our Terraform config since 0a96415 (modules/aws: configure
ELB idle timeout, coreos/tectonic-installer#725) when we were using
classic load balancers and should have dropped it in 16dfbb3
(data/aws: use nlbs instead of elbs, 2018-11-01, #594).

[1]: hashicorp/terraform-provider-aws@d25a227#diff-f4b0dbdc7e3eede6ba70cd286c834f37R78

data/data/aws/vpc/master-elb.tf

@@ -4,7 +4,6 @@ resource "aws_lb" "api_internal" {
   subnets                          = local.private_subnet_ids
   internal                         = true
   enable_cross_zone_load_balancing = true
-  idle_timeout                     = 3600
 
   tags = merge(
     {
@@ -26,7 +25,6 @@ resource "aws_lb" "api_external" {
   subnets                          = local.public_subnet_ids
   internal                         = false
   enable_cross_zone_load_balancing = true
-  idle_timeout                     = 3600
 
   tags = merge(
     {