Remove unused local variables
Remove unused local variables

This change suppresses following warnings:

spec/signal_thread_spec.rb:42: warning: assigned but unused variable - t
spec/signal_thread_spec.rb:58: warning: assigned but unused variable - n
spec/signal_thread_spec.rb:83: warning: shadowing outer local variable - t
spec/signal_thread_spec.rb:73: warning: assigned but unused variable - t

fluent/serverengine