Simplify styling of quotes for hovered annotations

Annotation quotes now have a default color of `$grey-7`, so there is no
need to have an additional selector for when the annotation card is
hovered.

h/static/styles/annotation.scss

 @import "mixins/icons";
 
-.thread-list__card:hover {
-  .annotation-header__user {
-    color: $grey-7;
-  }
-
-  .annotation-quote {
-    border-left: $highlight 3px solid;
-    color: $grey-5;
-  }
+// Highlight quote of annotation whenever its thread is hovered
+.thread-list__card:hover .annotation-quote {
+  border-left: $highlight 3px solid;
+  color: $grey-5;
 }
 
 // When hovering a top-level annotation, show the footer in a hovered state.