nginx(role): simplify installation by removing version spec

We always just want the latest anyway and therefore don't use it, so no
need to keep the complexity introduced by that setting.
Also merge repo_setup and nginx_install task lists into one
nginx_install task list as keeping two files isn't necessary.
Finally improving naming a bit.

roles/nginx/meta/argument_specs.yaml

@@ -1,15 +1,6 @@
 argument_specs:
   main:
     options:
-      nginx__version_spec:
-        description: >-
-          The version specification to use for installing the `nginx` package. The
-          provided version specification will be used like the following: `nginx={{
-          nginx__version_spec }}*`. This makes it possible to e.g. specify
-          until a minor version (like `1.3.`) and then have patch versions be
-          installed automatically (like `1.3.1` and so on).
-        type: str
-        required: true
       nginx__deploy_redirect_conf:
         description: >-
           Whether or not to deploy a `redirect.conf` to