[Internal] Move TFSDK model template to universe (#4303)

## Changes
<!-- Summary of your changes that are easy to understand -->
In order to get ready for auto-generating resources, we're relocating
the templates for our plugin framework models to an internal repository.

## Tests
<!-- 
How is this tested? Please see the checklist below and also describe any
other relevant tests
-->
Verified that generating an SDK before and after this change resulted in
a no-op.

- [x] `make test` run locally
- [ ] relevant change in `docs/` folder
- [ ] covered with integration tests in `internal/acceptance`
- [ ] relevant acceptance tests are passing
- [ ] using Go SDK

Co-authored-by: Omer Lachish <[email protected]>

.codegen.json

@@ -1,9 +1,6 @@
 {
     "formatter": "make fmt",
     "mode": "tf_v1",
-    "packages": {
-        ".codegen/model.go.tmpl": "internal/service/{{.Name}}_tf/model.go"
-    },
     "changelog_config": ".codegen/changelog_config.yml",
     "version": {
         "common/version.go": "version = \"$VERSION\""