Remove the "Create account" banner

The text is potentially inappropriate in the context of
publisher-managed accounts where the account might not be free and the
user might not be able to create one themselves.

In the first party account context we agreed that the banner is a bit
redundant anyway. See
https://hypothes-is.slack.com/archives/C07NXBDNW/p1510080494000386

src/sidebar/templates/hypothesis-app.html

@@ -15,14 +15,6 @@
     on-change-sort-key="vm.setSortKey(sortKey)">
   </top-bar>
 
-  <div class="create-account-banner" ng-if="vm.isSidebar && vm.auth.status === 'logged-out'">
-    To annotate this document
-    <a href="{{ vm.serviceUrl('signup') }}" target="_blank">
-      create a free account
-    </a>
-    or <a href="" ng-click="vm.login()">log in</a>
-  </div>
-
   <div class="content">
     <sidebar-tutorial ng-if="vm.isSidebar"></sidebar-tutorial>
     <share-dialog

src/styles/app.scss

@@ -88,24 +88,6 @@ hypothesis-app {
 // Elements in root template (viewer.html)
 // ---------------------------------------
 
-.create-account-banner {
-  background-color: $gray-dark;
-  border-radius: 2px;
-  color: $color-silver-chalice;
-  font-weight: bold;
-  height: 34px;
-  line-height: 34px;
-  margin-bottom: .72em;
-  margin-left: auto;
-  margin-right: auto;
-  text-align: center;
-  width: 100%;
-}
-
-.create-account-banner a {
-  color: $white;
-}
-
 .sheet {
   border: solid 1px $gray-lighter;
   border-radius: 2px;