Update submit button label to "Add"

Remove prop because it no longer contains "related issue" in it
and probably doesn't need to be customized anymore

See https://gitlab.com/gitlab-org/gitlab-ce/issues/34336

app/assets/javascripts/issuable/related_issues/components/add_issuable_form.vue

@@ -12,10 +12,6 @@ export default {
       type: String,
       required: true,
     },
-    addButtonLabel: {
-      type: String,
-      required: true,
-    },
     pendingReferences: {
       type: Array,
       required: false,
@@ -144,7 +140,7 @@ export default {
         class="js-add-issuable-form-add-button btn btn-new pull-left"
         @click="onFormSubmit"
         :disabled="isSubmitButtonDisabled">
-        {{ addButtonLabel }}
+        Add
         <loadingIcon
           ref="loadingIcon"
           v-if="isSubmitting"

app/assets/javascripts/issuable/related_issues/components/related_issues_block.vue

@@ -141,7 +141,6 @@ export default {
           :is-submitting="isSubmitting"
           :input-value="inputValue"
           :pending-references="pendingReferences"
-          add-button-label="Add related issues"
           :auto-complete-sources="autoCompleteSources" />
       </div>
       <div

spec/javascripts/issuable/related_issues/components/add_issuable_form_spec.js

@@ -45,7 +45,6 @@ describe('AddIssuableForm', () => {
           vm = new AddIssuableForm({
             propsData: {
               inputValue: '',
-              addButtonLabel: 'Submit',
               pendingReferences: [],
             },
           }).$mount();
@@ -62,7 +61,6 @@ describe('AddIssuableForm', () => {
           vm = new AddIssuableForm({
             propsData: {
               inputValue: 'foo',
-              addButtonLabel: 'Submit',
               pendingReferences: [],
             },
           }).$mount();
@@ -76,13 +74,11 @@ describe('AddIssuableForm', () => {
 
     describe('with references', () => {
       const inputValue = 'foo #123';
-      const addButtonLabel = 'Add issuable';
 
       beforeEach(() => {
         vm = new AddIssuableForm({
           propsData: {
             inputValue,
-            addButtonLabel,
             pendingReferences: [
               issuable1.reference,
               issuable2.reference,
@@ -91,10 +87,6 @@ describe('AddIssuableForm', () => {
         }).$mount();
       });
 
-      it('should put button label in place', () => {
-        expect(vm.$refs.addButton.textContent.trim()).toEqual(addButtonLabel);
-      });
-
       it('should put input value in place', () => {
         expect(vm.$refs.input.value).toEqual(inputValue);
       });
@@ -113,7 +105,6 @@ describe('AddIssuableForm', () => {
         vm = new AddIssuableForm({
           propsData: {
             inputValue: '',
-            addButtonLabel: 'Submit',
             pendingReferences: [
               issuable1.reference,
               issuable2.reference,
@@ -153,7 +144,6 @@ describe('AddIssuableForm', () => {
       vm = new AddIssuableForm({
         propsData: {
           inputValue: '',
-          addButtonLabel: 'Add issuable',
           pendingIssuables: [
             issuable1,
           ],