Update db/structure.sql

* No changes directly relevant to bouncer
* Has new `host_paths.canonical_path`
Remove path_hash from bouncer tests

* Was required when it was part of a UNIQUE index
* Following deploy of
  https://github.com/alphagov/transition/commit/1b8887ba4f5d9150edb24e6ce6b0fe6a918922d2
  is not required
* No longer need digest/sha1
Merge pull request #107 from alphagov/remove-path-hash-references

Remove path hash references