tpool: remove m_group_enqueued (unused)

Noted in MDEV-36542 that it wasn't used and
the compile warning -Wunused-private-field is legitimate.

Review: Vladislav Vaintroub

Author: grooverdan

tpool/tpool_generic.cc

@@ -218,7 +218,6 @@ class thread_pool_generic : public thread_pool
 
   /** Overall number of enqueues*/
   unsigned long long m_tasks_enqueued;
-  unsigned long long m_group_enqueued;
   /** Overall number of dequeued tasks. */
   unsigned long long m_tasks_dequeued;