Thread pools: removed unused pointer to memory pool.

No functional changes.

src/core/ngx_thread_pool.c

diff --git a/src/core/ngx_thread_pool.c b/src/core/ngx_thread_pool.c
index 191566c..c385317 100644
--- a/src/core/ngx_thread_pool.c
+++ b/src/core/ngx_thread_pool.c
@@ -33,7 +33,6 @@
     ngx_thread_cond_t         cond;
 
     ngx_log_t                *log;
-    ngx_pool_t               *pool;
 
     ngx_str_t                 name;
     ngx_uint_t                threads;
@@ -129,7 +128,6 @@
     }
 
     tp->log = log;
-    tp->pool = pool;
 
     err = pthread_attr_init(&attr);
     if (err) {