Remove unused CSS classes in top-level pages

I couldn't work out how to generate these classes dynamically. I wanted
to use the page slug, but it didn't seem to be available. However since
it doesn't look as if they're being used, I've chosen to remove them for
now.

about.html

@@ -56,7 +56,7 @@ title: About
 
 
 
-  <div class="about">
+  <div>
 
     <div id="page-banner">
       <div class="row">

join.html

@@ -56,7 +56,7 @@ title: Join
 
 
 
-  <div class="join">
+  <div>
 
     <div id="page-banner">
       <div class="row">

manifesto.html

@@ -56,7 +56,7 @@ title: Manifesto
 
 
 
-  <div class="manifesto">
+  <div>
 
     <div id="page-banner">
       <div class="row">