Removed default background image for home and courses pages

lms/static/sass/base/_variables.scss

@@ -302,7 +302,7 @@ $homepage__header--background: lighten($blue, 15%);
 // ====================
 
 // IMAGES: backgrounds
-$homepage-bg-image: '../images/homepage-bg.jpg';
+$homepage-bg-image: none;
 
 $login-banner-image: url(../images/bg-banner-account.png);
 $register-banner-image: url(../images/bg-banner-account.png);

lms/static/sass/multicourse/_course_about.scss

@@ -5,7 +5,7 @@
 
   header.course-profile {
     background: $course-profile-bg;
-    @include background-image(url($homepage-bg-image));
+    background-image: $homepage-bg-image;
     background-size: cover;
     box-shadow: 0 1px 80px 0 rgba(0,0,0, 0.5);
     border-bottom: 1px solid $border-color-3;

lms/static/sass/multicourse/_courses.scss

@@ -5,7 +5,7 @@
   header.search {
     background: $course-profile-bg;
     background-size: cover;
-    @include background-image(url($homepage-bg-image));
+    background-image: $homepage-bg-image;
     background-position: center top !important;
     border-bottom: 1px solid $border-color-3;
     box-shadow: inset 0 -1px 8px 0 rgba(0,0,0, 0.2), inset 0 1px 12px 0 rgba(0,0,0, 0.3);

lms/static/sass/multicourse/_home.scss

@@ -9,6 +9,7 @@
   > header {
     @include linear-gradient($homepage__header--gradient__color--alpha, $homepage__header--gradient__color--bravo);
     background-size: cover;
+    background-image: $homepage-bg-image;
     border-bottom: 1px solid $border-color-3;
     box-shadow: 0 1px 0 0 $course-header-bg, inset 0 -1px 5px 0 rgba(0,0,0, 0.1);
     @include clearfix;