Remove unused QA selectors

These selectors were left behind when selectors only used in units
tests were being renamed.

app/assets/javascripts/vue_shared/components/markdown/header.vue

@@ -124,7 +124,7 @@ export default {
               :cursor-offset="4"
               :tag-content="lineContent"
               icon="doc-code"
-              class="qa-suggestion-btn js-suggestion-btn"
+              class="js-suggestion-btn"
               @click="handleSuggestDismissed"
             />
             <gl-popover

app/assets/javascripts/vue_shared/components/markdown/suggestion_diff_header.vue

@@ -55,7 +55,7 @@ export default {
     <gl-button
       v-else-if="canApply"
       v-gl-tooltip.viewport="__('This also resolves the discussion')"
-      class="btn-inverted qa-apply-btn js-apply-btn"
+      class="btn-inverted js-apply-btn"
       :disabled="isApplying"
       variant="success"
       @click="applySuggestion"