Remove "show_unanchored_annotations" feature

We've discussed this feature and decided that it can't be turned on in
its current state. This commit removes it.

Note that the feature flag in `h/features.py` remains until these
changes are deployed out to production.

h/static/scripts/directive/test/thread-test.coffee

@@ -132,10 +132,6 @@ describe 'thread', ->
        it 'returns false', ->
          assert.isFalse(controller.shouldShow())

        it 'returns true if show_unanchored_annotations is on', ->
          $scope.feature = -> true
          assert.isTrue(controller.shouldShow())

      describe 'when the thread filter is active', ->
        beforeEach ->
          controller.filter = {active: -> true}

h/static/scripts/directive/thread.coffee

@@ -59,9 +59,7 @@ ThreadController = [
        return false

      if this.container?.message?.$orphan == true
        # Hide unless show_unanchored_annotations is turned on
        if not $scope.feature('show_unanchored_annotations')
          return false
        return false

      editing = this._count('edit') > 0
      matching = this._count('match') > 0

h/static/styles/threads.scss

@@ -56,11 +56,3 @@ $thread-padding: $annotation-card-left-padding;
.thread-load-more {
  clear: both;
}

.thread-anchor-notice {
  // XXX: negative margins here make me sad; maybe refactor container
  background-color: lighten($gray-light, 38%);
  border-top: solid 1px $gray-lighter;
  margin: 0 -1em -1em -1em;
  padding: 1em;
}

h/templates/client/thread.html

@@ -46,8 +46,3 @@
      ng-show="vm.shouldShowAsReply()">
  </li>
</ul>

<footer class="thread-anchor-notice"
        ng-if="feature('show_unanchored_annotations') && isSidebar && vm.container.message.$orphan">
  <em>We can't find the exact position of this annotation.</em>
</footer>