Revert "Workaround ruby/prism#2289"

This reverts commit 6769f95.

Author: eregon

test/mri/tests/ruby/test_process.rb

@@ -1823,12 +1823,7 @@ def test_system_sigpipe
       end
     end
   ensure
-    # https://github.com/ruby/prism/issues/2289
-    # Process.kill(:KILL, pid) if (pid != 0) rescue false
-    begin
-      Process.kill(:KILL, pid) if (pid != 0)
-    rescue Errno::ESRCH
-    end
+    Process.kill(:KILL, pid) if (pid != 0) rescue false
   end
 
   if Process.respond_to?(:daemon)