Reorganize rules in timdexui.scss

Why these changes are being introduced:

There are some CSS rules in `timdexui.scss` with a note that
they should be moved out of a file if there's a better place to
put them. Since then, we've added several partials that might make
logical sense to contain to these rules.

Relevant ticket(s):

N/A.

How this addresses that need:

This makes the following changes to `timdexui.scss`:

* The `#advanced-search-panel legend` rule moves to the `panels`
partial. (Though, it would also make sense in the `search` partial.)
* The `.top-space` rule moves to the `results` partial, as it is only
used for the filter container spacing.
* The `.top-attached` and `.top-line` rules are removed. Currently,
`.top-line` is only used in the style guide layout, and
`.top-attached` appears not to be used anywhere.

Side effects of this change:

The style guide layout will no longer have a top border.

Author: jazairi

app/assets/stylesheets/partials/_panels.scss

@@ -72,3 +72,7 @@
     }
   }
 }
+
+#advanced-search-panel legend {
+  color: #000
+}

app/assets/stylesheets/partials/_results.scss

@@ -1,3 +1,7 @@
+.top-space {
+  margin-top: 2.4rem;
+}
+
 .wrap-results {
   margin-bottom: 3rem;
   background-color: $white-t;

app/assets/stylesheets/timdexui.scss

@@ -1,20 +1,3 @@
 body {
   font-family: 'Roboto', 'Helvetica Neue', Helvetica, Arial, sans-serif;
 }
-
-// Things I don't know how to classify yet. Ideally this will be empty.
-.top-line {
-  border-top: 1px solid #000;
-}
-
-.top-attached {
-  margin-top: 0;
-}
-
-.top-space {
-  margin-top: 2.4rem;
-}
-
-#advanced-search-panel legend {
-  color: #000
-}