test_signal.rb: remove stale test

* test/ruby/test_signal.rb (TestSignal#test_trap_system_default):
  remove stale test.  signals are delivered to the main thread
  always now, so sleep in sub thread is no longer interrupted.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47665 b2dd03c8-39d4-4d8f-98ff-823fe69b080e