[GDBserver] Remove td_ta_event_addr td_ta_set_event and td_ta_event_getmsg
    
    As a result of this commit,
    
      9b4c5f878ff39e04127a1ad95f6b3832afe6d278
      (Remove support for thread events without PTRACE_EVENT_CLONE in GDBServer.)
    
    the last usage of td_ta_event_addr td_ta_set_event and
    td_ta_event_getmsg were removed.  They are no longer used.  This patch
    is to remove them.
    
    gdb/gdbserver:
    
    2016-08-02  Yao Qi  <yao.qi@linaro.org>
    
    	* thread-db.c (struct thread_db) <td_ta_event_getmsg_p>: Remove.
    	<td_ta_set_event_p, td_ta_event_addr_p>: Remove.
    	(thread_db_load_search): Update.
    	(try_thread_db_load_1): Don't look for td_ta_event_addr,
    	td_ta_set_event and td_ta_event_getmsg.