MC-4116: Update slider to use new background images logic

- remove unnecessary styles and nesting

Author: anthoula

app/code/Magento/PageBuilder/view/adminhtml/web/css/source/content-type/banner/_collage-left.less

@@ -10,5 +10,4 @@
 @import './_default.less';
 @import './_poster.less';
 @import './_collage-center';
-@import './_collage-left.less';
 @import './_collage-right.less';

app/code/Magento/PageBuilder/view/adminhtml/web/css/source/content-type/banner/_import.less

@@ -7,11 +7,9 @@
 //  Collage left appearance styles
 //  _____________________________________________
 
-.pagebuilder-slide {
-    .pagebuilder-overlay {
-        -moz-transition: background-color 500ms ease;
-        -o-transition: background-color 500ms ease;
-        -webkit-transition: background-color 500ms ease;
-        transition: background-color 500ms ease;
-    }
+.pagebuilder-slide .pagebuilder-overlay {
+    -moz-transition: background-color 500ms ease;
+    -o-transition: background-color 500ms ease;
+    -webkit-transition: background-color 500ms ease;
+    transition: background-color 500ms ease;
 }