Remove redundant initialization of global InnoDB variables

Also, remove the unused global variable srv_priority_boost.

storage/innobase/include/srv0srv.h

@@ -516,7 +516,6 @@ extern ulong	srv_n_spin_wait_rounds;
 extern ulong	srv_n_free_tickets_to_enter;
 extern ulong	srv_thread_sleep_delay;
 extern uint	srv_spin_wait_delay;
-extern ibool	srv_priority_boost;
 
 extern ulint	srv_truncated_status_writes;
 /** Number of initialized rollback segments for persistent undo log */