Remove comment for pre-Rails 3.2 code

Corresponding code was removed in 689c76f.

Author: bquorning

lib/active_record_shards/association_collection_connection_selection.rb

@@ -32,8 +32,6 @@ def initialize(association_collection, which)
       end
 
       def method_missing(method, *args, &block) # rubocop:disable Style/MethodMissing
-        # would love to not rely on version here, unfortunately @association_collection
-        # is a sensitive little bitch of an object.
         reflection = @association_collection.proxy_association.reflection
         reflection.klass.on_cx_switch_block(@which) { @association_collection.send(method, *args, &block) }
       end