Use Bootstrap Flex utility to remove .d-flex-center

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

ee/app/assets/javascripts/vue_merge_request_widget/components/approvals/multiple_rule/approvals_footer.vue

@@ -59,7 +59,7 @@ export default {
   <div>
     <div class="mr-widget-extension d-flex align-items-center pl-3">
       <button
-        class="btn btn-blank square s32 d-flex-center append-right-default"
+        class="btn btn-blank square s32 append-right-default"
         type="button"
         :aria-label="ariaLabel"
         @click="toggle"

ee/app/assets/javascripts/vue_merge_request_widget/components/approvals/multiple_rule/approvals_summary_optional.vue

@@ -39,7 +39,7 @@ export default {
       :href="helpPath"
       :title="__('About this feature')"
       target="_blank"
-      class="d-flex-center pl-1"
+      class="pl-1"
     >
       <icon name="question-o" />
     </gl-link>