Revert "Enhance bridge for better target merging"

This reverts commit d3298324561a11e68f2690ee2863f8dbab17db2a.

Conflicts:
	h/static/scripts/plugin/bridge.coffee

Fix #1020
Remove dead code

The `highlights` stub is no longer needed in the sidebar because
with the new highlights plugins that @csillag made the sidebar no
longer has any code that accesses this property.
Remove unused target ignore

The guest frames should not be overwriting the targets when they
fail to anchor an annotation. Instead, the anchors property is
used now to store the realized anchors from the target data.
Merge pull request #1500 from hypothesis/1020-bridge-races

Clean up unused code for bridge race conditions