remove duplicated css code, fixes #2439

Closes #2439

Merge request studip/studip!1633


(cherry picked from commit 79e20d95)

resources/assets/stylesheets/mixins/studip.less

@@ -166,14 +166,6 @@
 .scrollbox-vertical {
     overflow: auto;
 
-    background:
-        /* Shadow covers */
-        linear-gradient(white 30%, rgba(255,255,255,0)),
-        linear-gradient(rgba(255,255,255,0), white 70%) 0 100%,
-
-        /* Shadows */
-        radial-gradient(farthest-side at 50% 0, rgba(0,0,0,.2), rgba(0,0,0,0)),
-        radial-gradient(farthest-side at 50% 100%, rgba(0,0,0,.2), rgba(0,0,0,0)) 0 100%;
     background:
         /* Shadow covers */
         linear-gradient(white 30%, rgba(255,255,255,0)),
@@ -192,14 +184,6 @@
 .scrollbox-horizontal {
     overflow: auto;
 
-    background:
-        /* Shadow covers */
-        linear-gradient(90deg, white 30%, rgba(255,255,255,0)),
-        linear-gradient(90deg, rgba(255,255,255,0), white 70%) 100% 0,
-
-        /* Shadows */
-        radial-gradient(farthest-side at 0 50%, rgba(0,0,0,.2), rgba(0,0,0,0)),
-        radial-gradient(farthest-side at 100% 50%, rgba(0,0,0,.2), rgba(0,0,0,0)) 100% 0;
     background:
         /* Shadow covers */
         linear-gradient(90deg, white 30%, rgba(255,255,255,0)),

resources/assets/stylesheets/mixins/studip.scss

@@ -182,14 +182,6 @@
 %scrollbox-vertical {
     overflow: auto;
 
-    background:
-        /* Shadow covers */
-        linear-gradient(white 30%, rgba(255,255,255,0)),
-        linear-gradient(rgba(255,255,255,0), white 70%) 0 100%,
-
-        /* Shadows */
-        radial-gradient(farthest-side at 50% 0, rgba(0,0,0,.2), rgba(0,0,0,0)),
-        radial-gradient(farthest-side at 50% 100%, rgba(0,0,0,.2), rgba(0,0,0,0)) 0 100%;
     background:
         /* Shadow covers */
         linear-gradient(white 30%, rgba(255,255,255,0)),
@@ -208,14 +200,6 @@
 @mixin scrollbox-horizontal {
     overflow: auto;
 
-    background:
-        /* Shadow covers */
-        linear-gradient(90deg, white 30%, rgba(255,255,255,0)),
-        linear-gradient(90deg, rgba(255,255,255,0), white 70%) 100% 0,
-
-        /* Shadows */
-        radial-gradient(farthest-side at 0 50%, rgba(0,0,0,.2), rgba(0,0,0,0)),
-        radial-gradient(farthest-side at 100% 50%, rgba(0,0,0,.2), rgba(0,0,0,0)) 100% 0;
     background:
         /* Shadow covers */
         linear-gradient(90deg, white 30%, rgba(255,255,255,0)),