Don't wrap loop around waitpid call

What happens in #485 is that the loop keeps running until it's killed by
puma with a `KILL` signal. But that loop is actually not needed.

Author: st0012

lib/debug/local.rb

@@ -98,7 +98,7 @@ def after_fork_parent
           # only check child process from its parent
           begin
             # wait for all child processes to keep terminal
-            loop{ Process.waitpid }
+            Process.waitpid
           rescue Errno::ESRCH, Errno::ECHILD
           end
         end