Remove button padding override

Not sure why we were doing this. But as part of the Elements upgrade
our back buttons got bigger (because they weren’t constrained by the
line in our `app.scss`) but any green submit buttons stayed smaller in
height, so didn’t line up.

This commit removes the override, so all buttons have the size that
Elements intended.

app/assets/stylesheets/app.scss

@@ -154,10 +154,6 @@ td {
   color: $secondary-text-colour;
 }
 
-.button {
-  padding: 0.3em 0.6em 0.2em 0.6em;
-}
-
 .list-bullet {
 
   .list-bullet {