Remove logstash-forwarder from integration testing

[andsel]

Logstash-forwarder has been completely supplanted by Beats and ElasticAgent, so there is no need to the keep testing on it like in https://github.com/logstash-plugins/logstash-input-beats/blob/339913cbf0268da82ab517cfdbf5e3c3ece99191/spec/integration/logstash_forwarder_spec.rb .

• remove the testing code being sure that there isn't any loose in code coverage terms, that all meaningful source paths for Beats keeps tested.
• update the fixture preparation test script to don't download it:

  logstash-input-beats/lib/tasks/test.rake

  Lines 34 to 43 in 339913c

  	desc "Download latest stable version of Logstash-forwarder"
  	task :lsf do
  	destination = File.join(VENDOR_PATH, "logstash-forwarder")
  	FileUtils.rm_rf(destination)
  	FileUtils.mkdir_p(destination)
  	download_destination = File.join(destination, "logstash-forwarder")
  	puts "Logstash-forwarder: downloading from #{LSF_URL} to #{download_destination}"
  	download(LSF_URL, download_destination)
  	File.chmod(0755, download_destination)
  	end