Retire TopLevelDescribe

Why?

 - it was slow rubocop/rubocop-rspec#925 (comment)
 - it ignored non-describe top-level example groups rubocop/rubocop-rspec#925 (comment)

`TopLevelGroup` is a modern replacement for `TopLevelDescribe`.

Examples how to migrate cops from TopLevelDescribe to TopLevelGroup:

 - rubocop/rubocop-rspec#932
 - rubocop/rubocop-rspec#977

lib/rubocop-capybara.rb

@@ -9,7 +9,6 @@
 require_relative 'rubocop/rspec/version'
 require_relative 'rubocop/rspec/inject'
 require_relative 'rubocop/rspec/node'
-require_relative 'rubocop/rspec/top_level_describe'
 require_relative 'rubocop/rspec/wording'
 require_relative 'rubocop/rspec/language'
 require_relative 'rubocop/rspec/language/node_pattern'