Don't require `API::API` in routes
The commit and the explanation from https://gitlab.com/gitlab-org/gitlab-ce/issues/18312#note_19895106 should be enough to justify this change:
Don't require `API::API` in routes
`API::API` is autoloaded and shouldn't be required in non-autoloaded
code. Otherwise, we get the following dreaded error:
A copy of API::Helpers has been removed from the module tree but is
still active!
Closes #18312 (closed).