Fix failed spec in CI using jruby

```
An error occurred while installing rbs (3.5.1), and Bundler cannot continue.

In Gemfile:
  steep was resolved to 1.7.1, which depends on
    rbs
Error: The process '/home/runner/.rubies/jruby-9.4.7.0/bin/bundle' failed with exit code 5
```

See <https://github.com/GrottoPress/envy.rb/actions/runs/9555823265/job/26339829578>.

Gemfile

@@ -11,7 +11,6 @@ group :development, :test do
 end
 
 group :development do
-  gem "rbs", "~> 3.5", require: false
   gem "steep", "~> 1.7", require: false
 end