Correctly remove the buildin rails adapter

`defined?` for a string literal is always truthy, same as for symbols.
`defined?(ActiveJob::QueueAdapters::SneakersAdapter)` would work as an alternative.

```rb
module ActiveJob
  module QueueAdapters
    # Adapter removed from rails
  end
end

module ActiveJob
  module QueueAdapters
    remove_const(:SneakersAdapter) if defined?("::#{name}::SneakersAdapter")
  end
end
```

test.rb:10:in `remove_const': constant ActiveJob::QueueAdapters::SneakersAdapter not defined (NameError)

lib/active_job/queue_adapters/sneakers_adapter.rb

@@ -2,7 +2,7 @@
   module ActiveJob
     module QueueAdapters
       # Explicitly remove the implementation existing in older Rails versions'.
-      remove_const(:SneakersAdapter) if defined?("::#{name}::SneakersAdapter")
+      remove_const(:SneakersAdapter) if const_defined?(:SneakersAdapter)
 
       # = Sneakers adapter for Active Job
       #