Clean up unused code for bridge race conditions

Back in January, when #1020 was opened, I think some things were very different.

- The core annotator used the `highlights` property more. Now only the plugins do and they aren't loaded in the sidebar.
- The `setupAnnotation` function clobbered existing targets if they weren't re-discovered. I don't think that happens anymore.

So I removed all the code in d3298324561a11e68f2690ee2863f8dbab17db2a.

@csillag this one needs your review.

Combined with the changes of #1436 I don't see any issues with the bridge anymore.