Remove the custom headers from the `/api/` and `/api/links` API requests
so that we can preload these routes very early in the sidebar app's
startup process using `<link rel="preload">`, thus reducing the number
of roundtrips required before showing annotations.

The original motivations for adding this header (see [1]) don't really
apply to these trivial API requests but to ones which result in real
work on the backend.

For context, see https://hypothes-is.slack.com/archives/C4K6M7P5E/p1575447372326000.

[1] See https://github.com/hypothesis/client/pull/930.