rendered_markdown: Remove expensive :first-child and :last-child selectors.

With the refactoring of the rendered-Markdown area to use only
margin bottom, including in message-edit previewsk, these expensive,
general selectors are unnecessary.

Headings and horizontal rules, which do have margin-top, are zeroed
out elsewhere in the rendered-markdown file.

web/styles/compose.css

@@ -826,7 +826,7 @@
 
 textarea.new_message_textarea {
     display: table-cell;
-    padding: 5px;
+    padding: 5px 5px 0;
     height: 1.5em;
     max-height: 22em;
     margin: 0;
@@ -1359,7 +1359,7 @@ textarea.new_message_textarea {
 }
 
 .preview_message_area {
-    padding: 5px;
+    padding: 5px 5px 0;
     overflow: auto;
 
     border-radius: 3px 3px 0 0;

web/styles/message_row.css

@@ -696,7 +696,7 @@
     border: none;
     /* Match textarea padding to compose box and
        message-edit preview area. */
-    padding: 5px;
+    padding: 5px 5px 0;
     margin: 0;
 
     box-shadow: none;

web/styles/rendered_markdown.css

@@ -349,13 +349,6 @@
         }
     }
 
-    & > .spoiler-block:last-child {
-        /* A spoiler block at the end of a message, or as
-           a message's only content, gets the same bottom
-           margin as other elements. */
-        margin-bottom: var(--markdown-interelement-space-px);
-    }
-
     /* embedded link previews */
     .message_inline_image_title {
         font-weight: bold;
@@ -897,18 +890,6 @@
     }
 }
 
-.preview_content.rendered_markdown {
-    /*  Ensure that the first child and last child
-    don't have blank area at the top and bottom respectively. */
-    & > :first-child {
-        margin-top: 0;
-    }
-
-    & > :last-child:not(.message_inline_image) {
-        margin-bottom: 0;
-    }
-}
-
 .codehilite {
     display: block !important;
     border: none !important;