Remove parallel option from RuboCop

Since parallel analysis is enabled by default since RuboCop 1.19.0

https://github.com/rubocop/rubocop/releases/tag/v1.19.0
rubocop/rubocop#10000

Author: yahonda

.github/workflows/ruby.yml

@@ -34,4 +34,4 @@ jobs:
     - name: Build and run RuboCop
       run: |
         bundle install --jobs 4 --retry 3
-        bundle exec rubocop --parallel
+        bundle exec rubocop