make if condition more ruby-idiomatic

Co-Authored-By: Chalupa Petr <git@pitr.ch>

Author: fzakaria

lib/concurrent-ruby/concurrent/executor/ruby_thread_pool_executor.rb

@@ -209,9 +209,7 @@ def ns_assign_worker(*args, &task)
     #
     # @!visibility private
     def ns_enqueue(*args, &task)
-      if @synchronous
-        return false
-      end
+      return false if @synchronous
 
       if !ns_limited_queue? || @queue.size < @max_queue
         @queue << [task, args]