code: public void deleteBuildVariable(Integer projectId, String key)
              throws IOException {
          String tailUrl = GitlabProject.URL + "/" +
                  projectId +
                  GitlabBuildVariable.URL + "/" +
                  key;
          retrieve().method(DELETE).to(tailUrl, Void.class);
      }
docs: Deletes an existing variable.
  
  @param projectId The ID of the project containing the variable.
  @param key       The key of the variable to delete.
  @throws IOException on gitlab api call error