Remove an obsolete development task

This commit removes an obsolete `:generate_cops_documentation` development task.
It's been a year since it was introduced in rubocop@1905f1b
and is no longer in use.

tasks/cops_documentation.rake

@@ -14,8 +14,3 @@ task update_cops_documentation: :yard_for_generate_documentation do
   deps = %w[Bundler Gemspec Layout Lint Metrics Migration Naming Security Style]
   CopsDocumentationGenerator.new(departments: deps).call
 end
-
-desc 'Generate docs of all cops departments (obsolete)'
-task :generate_cops_documentation do
-  puts 'Updating the documentation is now done automatically!'
-end