Remove bogus and obsolete `--change-metadata-...` css mixins

Also fix that the strategy row was accidentally hidden for more than a
year. :-)

--change-metadata-assignee is obsolete and unused.

--change-metadata-strategy is only used by chrome-style to always hide,
but with the new metadata hiding this is taken care of in a better way.

--change-metadata-topic is unused.

Change-Id: I062d2704de05137739bcc8677f87b6ef98180c6f
diff --git a/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata_html.ts b/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata_html.ts
index 78e16fd..c080345 100644
--- a/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata_html.ts
+++ b/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata_html.ts
@@ -34,16 +34,6 @@
     .webLink {
       display: block;
     }
-    /* CSS Mixins should be applied last. */
-    section.assignee {
-      @apply --change-metadata-assignee;
-    }
-    section.strategy {
-      @apply --change-metadata-strategy;
-    }
-    section.topic {
-      @apply --change-metadata-topic;
-    }
     gr-account-chip[disabled],
     gr-linked-chip[disabled] {
       opacity: 0;
@@ -454,7 +444,6 @@
     <section
       class$="strategy [[_computeDisplayState(_showAllSections, change, _SECTION.STRATEGY)]]"
       hidden$="[[_computeHideStrategy(change)]]"
-      hidden=""
     >
       <span class="title">Strategy</span>
       <span class="value">[[_computeStrategy(change)]]</span>