[GDBserver] Remove td_ta_event_addr td_ta_set_event and td_ta_event_getmsg

[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.