Only accept strings for the logout_link_path configuration value

As of now we don't need to also allow procs or symbols as the logout
path is generally not dynamic.

admin/app/controllers/solidus_admin/auth.rb

@@ -22,7 +22,7 @@ def current_solidus_admin_user
   end
 
   def solidus_admin_logout_path
-    send SolidusAdmin::Config.logout_link_path
+    SolidusAdmin::Config.logout_link_path
   end
 
   def solidus_admin_logout_method

admin/lib/solidus_admin/configuration.rb

@@ -103,7 +103,7 @@ def main_nav
     preference :current_user_method, :string, default: :spree_current_user
 
     # The path used to logout the user in the admin interface.
-    preference :logout_link_path, :string, default: :admin_logout_path
+    preference :logout_link_path, :string, default: '/admin/logout'
 
     # The HTTP method used to logout the user in the admin interface.
     preference :logout_link_method, :string, default: :delete

admin/lib/solidus_admin/preview.rb

@@ -43,10 +43,6 @@ def authenticate_solidus_backend_user!
       # noop
     end
 
-    def admin_logout_path
-      "/logout"
-    end
-
     def current_component
       @current_component ||= begin
         # Lookbook sets the @preview instance variable with a PreviewEntity instance, while ViewComponent uses the preview class.

admin/spec/components/solidus_admin/sidebar/component_spec.rb

@@ -3,12 +3,7 @@
 require "spec_helper"
 
 RSpec.describe SolidusAdmin::Sidebar::Component, type: :component do
-  before do
-    allow(vc_test_controller).to receive(:spree_current_user).and_return(build(:user))
-    without_partial_double_verification do
-      allow(vc_test_controller.spree).to receive(:admin_logout_path).and_return('/logout')
-    end
-  end
+  before { allow(vc_test_controller).to receive(:spree_current_user).and_return(build(:user)) }
 
   it "renders the overview preview" do
     render_preview(:overview)