Remove duplicate rake development dependency

```
sidekiq-retries at * did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  duplicate dependency on rake (>= 0, development), (>= 0) use:
    add_runtime_dependency 'rake', '>= 0', '>= 0'
```