Remove dead singular assignee code in boards

Introduced in
https://gitlab.com/gitlab-org/gitlab-ce/commit/adc59353fa48c5e433b80efc1698c0b12e121968#852fdbe4145be33eca58d30d8c2f4a6623ef8602_30_30

Only usage removed in
https://gitlab.com/gitlab-org/gitlab-ce/commit/de9e559d4495560095adfa03ea6489ea51f6f2f4#e22df3c6b3631af90cde45bbb974ddce72aefab7_21_21

app/assets/javascripts/boards/components/board_sidebar.js

@@ -32,9 +32,6 @@ gl.issueBoards.BoardSidebar = Vue.extend({
     showSidebar () {
       return Object.keys(this.issue).length;
     },
-    assigneeId() {
-      return this.issue.assignee ? this.issue.assignee.id : 0;
-    },
     milestoneTitle() {
       return this.issue.milestone ? this.issue.milestone.title : 'No Milestone';
     }