Remove redundant background definitions

These elements didn't need a background because their parent elements
already defined the same background.

app/assets/stylesheets/layout.scss

@@ -746,7 +746,6 @@ body > header,
   flex-direction: column;
 
   @include breakpoint(medium) {
-    background: $body-background;
     flex-direction: row;
     padding-bottom: 0;
 
@@ -1761,8 +1760,6 @@ table {
 // ------------
 
 .comments {
-  background: $body-background;
-  background-repeat: repeat-x;
   padding-bottom: $line-height * 4;
 
   h2 {

app/assets/stylesheets/participation.scss

@@ -438,7 +438,6 @@
 }
 
 %panel {
-  background: $body-background;
   border: 1px solid;
   border-color: #e5e6e9 #dfe0e4 #d0d1d5;
   border-radius: 0;
@@ -1040,7 +1039,6 @@
 
 .jumbo-budget {
   @include full-width-border(bottom, 2px solid $border);
-  background: $body-background;
 
   &.budget-heading {
     min-height: $line-height * 10;