Simplify implementation of build retry service

app/services/ci/retry_build_service.rb

 module Ci
   class RetryBuildService < ::BaseService
     def execute(build)
-      # return unless build.retryable?
+      reprocess(build).tap do |new_build|
+        new_build.enqueue!
 
-      self.retry(build).tap do |new_build|
         MergeRequests::AddTodoWhenBuildFailsService
           .new(build.project, current_user)
           .close(new_build)
@@ -13,12 +13,6 @@ module Ci
       end
     end
 
-    def retry(build)
-      self.reprocess(build).tap do |new_build|
-        new_build.enqueue!
-      end
-    end
-
     def reprocess(build)
       unless can?(current_user, :update_build, build)
         raise Gitlab::Access::AccessDeniedError