remove mock allowances on nil objects (#4295)

Co-authored-by: Oleg Pudeyev <[email protected]>

Author: p-datadog

spec/datadog/tracing/contrib/rack/integration_test_spec.rb

@@ -58,8 +58,6 @@
           allow(Datadog::Core::Remote::Negotiation).to receive(:new).and_return(negotiation)
 
           allow(client).to receive(:id).and_return(remote_client_id)
-          allow(worker).to receive(:start).and_call_original
-          allow(worker).to receive(:stop).and_call_original
 
           Datadog.configure do |c|
             c.remote.enabled = remote_enabled