Remove unnecessary routing assertions require

ActionDispatch::Assertions has an autoload defined:
https://github.com/rails/rails/blob/v7.1.3/actionpack/lib/action_dispatch.rb#L130

That file requires action_dispatch/testing/assertions/routing.rb:
https://github.com/rails/rails/blob/v7.1.3/actionpack/lib/action_dispatch/testing/assertions.rb#L5

So ActionDispatch::Assertions::RoutingAssertions can be referenced
without being explicitly required.

rspec/rails/example/controller_example_group.rb, which is loaded before
this file, already references it and triggers the autoload, which means
this require never has any effect:
https://github.com/rspec/rspec-rails/blob/v6.1.1/lib/rspec/rails/example/controller_example_group.rb#L5

Requiring action_dispatch/testing/assertions/routing.rb on its own
doesn't actually work, because it autoloads ActionDispatch::Assertions
which then fails to find ActionDispatch::Assertions::RoutingAssertions
since it hasn't been defined yet:

    $ ruby -r action_dispatch -e "p ActionDispatch::Assertions::RoutingAssertions"
    ActionDispatch::Assertions::RoutingAssertions

    $ ruby -r action_dispatch -r action_dispatch/testing/assertions/routing -e "p ActionDispatch::Assertions::RoutingAssertions"
    /Users/eugene/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/actionpack-7.1.3/lib/action_dispatch/testing/assertions.rb:12:in `<module:Assertions>': uninitialized constant ActionDispatch::Assertions::RoutingAssertions (NameError)

Author: eugeneius

lib/rspec/rails/example/routing_example_group.rb

@@ -1,5 +1,3 @@
-require "action_dispatch/testing/assertions/routing"
-
 module RSpec
   module Rails
     # @private