Fixed style-lint errors for `panels.scss`

Removed @extend from SCSS file in favor of directly assigning CSS class
in corresponding HTML snippets.

app/assets/javascripts/related_merge_requests/components/related_merge_requests.vue

@@ -65,7 +65,7 @@ export default {
 
 <template>
   <div v-if="isFetchingMergeRequests || (!isFetchingMergeRequests && totalCount)">
-    <div id="merge-requests" class="card-slim mt-3">
+    <div id="merge-requests" class="card card-slim mt-3">
       <div class="card-header">
         <div class="card-title mt-0 mb-0 h5 merge-requests-title">
           <span class="mr-1">

app/assets/stylesheets/framework/panels.scss

@@ -3,7 +3,6 @@
 }
 
 .card-slim {
-  @extend .card;
   margin-bottom: $gl-vert-padding;
 }
 

app/views/projects/new.html.haml

@@ -51,7 +51,7 @@
             = render 'new_project_fields', f: f, project_name_id: "blank-project-name"
 
         #create-from-template-pane.tab-pane.js-toggle-container.px-0.pb-0{ class: active_when(active_tab == 'template'), role: 'tabpanel' }
-          .card-slim.m-4.p-4
+          .card.card-slim.m-4.p-4
             %div
               - contributing_templates_url = 'https://gitlab.com/gitlab-org/project-templates/contributing'
               - link_start = '<a href="%{url}" target="_blank" rel="noopener noreferrer">'.html_safe % { url: contributing_templates_url }