Simplify new investment layout

We had an additional `<div>` just to add a background color, when we can
do it by applying the background color to the whole `<main>` element and
then the body background color to the optional fields.

However, I've decided not to do so. The main purpose of changing the
background color is to highlight the required fields. The benefits of
changing the background color of the header as well are unclear. When in
doubt, we're using the solution which requires less code.

app/assets/stylesheets/budgets/investments/new.scss

@@ -2,13 +2,6 @@
   $border-width: 4px;
   @include grid-column-gutter;
 
-  > :first-child:not(.print-info) {
-    @include full-width-background($adjust-padding: true);
-    background: $light;
-    margin-top: -$line-height;
-    padding-top: $line-height;
-  }
-
   h1 {
     margin-bottom: 0;
   }
@@ -16,7 +9,6 @@
   header {
     @include has-fa-icon(building, regular, after);
     align-items: center;
-    background-color: $body-background;
     border: $border-width solid;
     color: $brand-secondary;
     border-bottom-right-radius: rem-calc(12);