CNG: Remove things that Ruby required in Gitaly image

#4398 (closed) aims to remove Ruby from the gitaly build image. There are a number of things that can also be removed once the sidecar disappears. This issue will enumerate all of those items.

• A directory where the ruby worker process lives, as configured under [gitaly-ruby][dir] in the config.toml.
• Ruby sentry DSN is no longer necessary, as configured under [logging][ruby_sentry_dsn] in the config.toml.
• We will need to make changes to charts/gitlab/charts/gitaly/templates/_configmap_spec.yaml, as config.toml.erb must be converted to config.toml.tpl. More details can be found in https://docs.gitlab.com/charts/development/style_guide.html#templating-configuration-files