Post PR #88 Cleanup

We can remove this fallback key for reading the repository to publish to now that the following PRs have been merged:

- heroku/buildpacks-go#124
- heroku/buildpacks-php#15
- heroku/buildpacks-ruby#184
- heroku/buildpacks-jvm#555
- heroku/buildpacks-nodejs#610
- heroku/buildpacks-python#58
- heroku/buildpacks-procfile#156

src/buildpacks.rs

@@ -37,14 +37,7 @@ pub(crate) fn read_image_repository_metadata(
  metadata
  .as_ref()
  .and_then(|metadata| metadata.get("release").and_then(|value| value.as_table()))
- .and_then(|release| {
- release
- .get("image")
- // TODO: remove this once each buildpack.toml metadata is updated to
- // replace [metadata.release.docker] with [metadata.release.image]
- .or_else(|| release.get("docker"))
- .and_then(|value| value.as_table())
- })
+ .and_then(|release| release.get("image").and_then(|value| value.as_table()))
  .and_then(|docker| docker.get("repository").and_then(|value| value.as_str()))
  .map(|value| value.to_string())
 }
@@ -64,30 +57,6 @@ mod test {
  use libcnb_data::buildpack::BuildpackDescriptor;
  use libcnb_package::GenericMetadata;
 
- #[test]
- fn test_read_image_repository_metadata_deprecated() {
- let data = r#"
-api = "0.9"
-
-[buildpack]
-id = "foo/bar"
-version = "0.0.1"
-
-[[stacks]]
-id = "*"
-
-[metadata.release.docker]
-repository = "deprecated repository value"
-"#;
-
- let buildpack_descriptor =
- toml::from_str::<BuildpackDescriptor<GenericMetadata>>(data).unwrap();
- assert_eq!(
- read_image_repository_metadata(&buildpack_descriptor),
- Some("deprecated repository value".to_string())
- );
- }
-
  #[test]
  fn test_read_image_repository_metadata() {
  let data = r#"