Standardise on avoiding except lists for check_api_readable

Although this is technically duplicative, it's much easier to read
and therefore to maintain, particularly if you put the _readable one
first.

app/controllers/api/traces_controller.rb

@@ -1,6 +1,6 @@
 module Api
   class TracesController < ApiController
-    before_action :check_api_readable, :only => [:show, :data]
+    before_action :check_api_readable
     before_action :check_api_writable, :only => [:create, :update, :destroy]
     before_action :set_locale
     before_action :authorize