Remove useless condition check

Since threads count is already guaranteed to be less or equal than
max_active_threads in thread_pool.rb:line 113, there is no need to
double check.

Not to mention that this will cause a race condition on the JRuby
platform, as noted in 5ac9df0.

Removing the conditional check will also reduce lock acquisitions
and improve overall efficiency.

In the original commit 1737459,
this check was essential. However, after subsequent changes,
it seems no one performed a careful review of this logic.

Author: DormancyWang

lib/rake/thread_pool.rb

@@ -108,19 +108,13 @@ def process_queue_item      #:nodoc:
       false
     end
 
-    def safe_thread_count
-      @threads_mon.synchronize do
-        @threads.count
-      end
-    end
-
     def start_thread # :nodoc:
       @threads_mon.synchronize do
         next unless @threads.count < @max_active_threads
 
         t = Thread.new do
           begin
-            while safe_thread_count <= @max_active_threads
+            loop do
               break unless process_queue_item
             end
           ensure