Remove unused issue_assignee partial

The usage for this partial was removed with:
95e04916

app/views/shared/issuable/form/_issue_assignee.html.haml

-- issue = issuable
-- assignees = issue.assignees
-.block.assignee
-  .sidebar-collapsed-icon.sidebar-collapsed-user{ data: { toggle: "tooltip", placement: "left", container: "body" }, title: (issuable.assignee_list) }
-    - if assignees.any?
-      - assignees.each do |assignee|
-        = link_to_member(@project, assignee, size: 24)
-    - else
-      = icon('user', 'aria-hidden': 'true')
-  .title.hide-collapsed
-    Assignee
-    = icon('spinner spin', class: 'hidden block-loading', 'aria-hidden': 'true')
-    - if can_edit_issuable
-      = link_to 'Edit', '#', class: 'js-sidebar-dropdown-toggle edit-link float-right'
-  .value.hide-collapsed
-    - if assignees.any?
-      - assignees.each do |assignee|
-        = link_to_member(@project, assignee, size: 32, extra_class: 'bold') do
-          %span.username
-            = assignee.to_reference
-    - else
-      %span.assign-yourself.no-value
-        No assignee
-        - if can_edit_issuable
-          \-
-          %a.js-assign-yourself{ href: '#' }
-            assign yourself
-
-  .selectbox.hide-collapsed
-    = f.hidden_field 'assignee_ids', value: issuable.assignee_ids, id: 'issue_assignee_ids'
-    = dropdown_tag('Select assignee', options: { toggle_class: 'js-user-search js-author-search', title: 'Assign to', filter: true, dropdown_class: 'dropdown-menu-user dropdown-menu-selectable dropdown-menu-author', placeholder: 'Search users', data: { first_user: (current_user.username if current_user), current_user: true, project_id: (@project.id if @project), author_id: issuable.author_id, field_name: "#{issuable.to_ability_name}[assignee_id]", issue_update: issuable_json_path(issuable), ability_name: issuable.to_ability_name, null_user: true } })