Remove asset_dir variable and optional asset writes

* Originally, poseidon/terraform-render-bootstrap generated
TLS certificates, manifests, and cluster "assets" written
to local disk (`asset_dir`) during terraform apply cluster
bootstrap
* Typhoon v1.17.0 introduced bootstrapping using only Terraform
state to store cluster assets, to avoid ever writing sensitive
materials to disk and improve automated use-cases. `asset_dir`
was changed to optional and defaulted to "" (no writes)
* Typhoon v1.18.0 deprecated the `asset_dir` variable, removed
docs, and announced it would be deleted in future.
* Add Terraform output `assets_dir` map
* Remove the `asset_dir` variable

Cluster assets are now stored in Terraform state only. For those
who wish to write those assets to local files, this is possible
doing so explicitly.

```
resource local_file "assets" {
  for_each = module.yavin.assets_dist
  filename = "some-assets/${each.key}"
  content = each.value
}
```

Related:

* https://github.com/poseidon/typhoon/pull/595
* https://github.com/poseidon/typhoon/pull/678

aws/container-linux/kubernetes/variables.tf

@@ -149,12 +149,6 @@ variable "worker_node_labels" {
 
 # unofficial, undocumented, unsupported
 
-variable "asset_dir" {
-  type        = string
-  description = "Absolute path to a directory where generated assets should be placed (contains secrets)"
-  default     = ""
-}
-
 variable "cluster_domain_suffix" {
   type        = string
   description = "Queries for domains with the suffix will be answered by CoreDNS. Default is cluster.local (e.g. foo.default.svc.cluster.local)"