Remove unused installer template bindings

[rhcarvalho]

Following up on unifying .gitignore templates into a static file, an audit of all installer bindings was conducted to see if any variables eliminated from .gitignore.eex became unused repo-wide.

While all variables from .gitignore.eex remain required by other templates, the audit uncovered two pre-existing unused bindings (pubsub_server and root_app_name), allowing us to clean them up along with their supporting code:

• pubsub_server: Stopped being used in templates in commit 04398b6 (June 2019) when PubSub was moved into the application supervision tree and templates switched to referencing <%= web_namespace %>.PubSub directly. Remove pubsub_server from binding and delete the helper get_pubsub_server/1.

• root_app_name / root_app: root_app_name was defined when umbrella generators were added in commit dad413f (Jan 2017), but templates have always used @root_app_module (MyApp.Umbrella) instead. Remove root_app_name from binding and drop the root_app struct field from %Phx.New.Project{}.

• Helper cleanup: Project.new/2 already initializes root_mod to app_mod (Module.concat([opts[:module] || Macro.camelize(app)])). In Phx.New.Single, put_root_app/1 was computing the exact same value a second time to assign root_mod, making it a no-op. Remove put_root_app/1 from Single, and rename put_root_app/1 to put_root_mod/1 in Phx.New.Umbrella where it actually overrides root_mod with the MyApp.Umbrella module name.