Remove SCSS extends from pipeline page bundles

Also remove description block, it was removed in 8.5,
according to the output of:
git log -S description-block master --source

app/assets/stylesheets/framework/common.scss

@@ -269,12 +269,6 @@ img.emoji {
   height: 220px;
 }
 
-.description-block {
-  @extend .light-well;
-  @extend .light;
-  margin-bottom: 10px;
-}
-
 .footer-links {
   margin-bottom: 20px;
 

app/assets/stylesheets/page_bundles/_pipeline_mixins.scss

@@ -176,12 +176,6 @@
   li {
     position: relative;
 
-    // ensure .mini-pipeline-graph-dropdown-item has hover style when action-icon is hovered
-    &:hover > .mini-pipeline-graph-dropdown-item,
-    &:hover > .ci-job-component > .mini-pipeline-graph-dropdown-item {
-      @extend .mini-pipeline-graph-dropdown-item:hover;
-    }
-
     // link to the build
     .mini-pipeline-graph-dropdown-item {
       align-items: center;
@@ -216,13 +210,16 @@
           display: block;
         }
       }
+    }
 
-      &:hover,
-      &:focus {
+    // ensure .mini-pipeline-graph-dropdown-item has hover style when action-icon is hovered
+    &:hover > .mini-pipeline-graph-dropdown-item,
+    &:hover > .ci-job-component > .mini-pipeline-graph-dropdown-item,
+    .mini-pipeline-graph-dropdown-item:hover,
+    .mini-pipeline-graph-dropdown-item:focus {
       outline: none;
       text-decoration: none;
       background-color: var(--gray-100, $gray-50);
     }
   }
-  }
 }

app/assets/stylesheets/page_bundles/pipeline.scss

@@ -306,11 +306,6 @@
       }
     }
 
-    // ensure .build-content has hover style when action-icon is hovered
-    .ci-job-dropdown-container:hover .build-content {
-      @extend .build-content:hover;
-    }
-
     .ci-status-icon svg {
       height: 24px;
       width: 24px;
@@ -330,6 +325,7 @@
       @include build-content();
     }
 
+    .ci-job-dropdown-container:hover .build-content,
     a.build-content:hover,
     button.build-content:hover {
       background-color: var(--gray-100, $gray-100);