Retire TopLevelDescribe

Why?

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

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

Examples how to migrate cops from TopLevelDescribe to TopLevelGroup:

 - #932
 - #977

Author: pirj

lib/rubocop-rspec.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'