Remove support for thread events without PTRACE_EVENT_CLONE in GDB

Before, on systems that did not support PTRACE_EVENT_CLONE, both GDB and
GDBServer coordinated with libthread_db.so to insert breakpoints at magic
locations in libpthread.so, in order to break at thread creation and
thread death.

Support for thread events was removed from GDBServer as patch:
https://sourceware.org/ml/gdb-patches/2015-11/msg00466.html

This patch removes support for thread events in GDB.

No regressions found on Ubuntu 14.04 x86_64.

gdb/ChangeLog:

	* breakpoint.c (remove_thread_event_breakpoints): Remove.
	* breakpoint.h (remove_thread_event_breakpoints): Remove
	declaration.
	* linux-nat.c (in_pid_list_p): Remove.
	(lin_lwp_attach_lwp): Remove.
	* linux-nat.h (lin_lwp_attach_lwp): Remove declaration.
	* linux-thread-db.c (thread_db_use_events): Remove.
	(struct thread_db_info) <td_create_bp_addr>: Remove.
	<td_death_bp_addr>: Likewise.
	<td_ta_event_addr_p>: Likewise.
	<td_ta_set_event_p>: Likewise.
	<td_ta_clear_event_p>: Likewise.
	<td_ta_event_getmsg_p>: Likewise.
	<td_thr_event_enable_p>: Likewise.
	(attach_thread): Likewise.
	(detach_thread): Likewise.
	(have_threads_callback): Likewise.
	(have_threads): Likewise.
	(enable_thread_event): Likewise.
	(enable_thread_event_reporting): Likewise.
	(try_thread_db_load_1): Remove td_ta_event_addr, td_ta_set_event,
	td_ta_clear_event, td_ta_event_getmsg, td_thr_event_enable
	initializations.
	(try_thread_db_load_1): Remove enable_thread_event_reporting call.
	(disable_thread_event_reporting): Remove.
	(record_thread): Adapt to thread_db_use_event removal.
	(detach_thread): Remove.
	(thread_db_detach): Adapt to thread_db_use_event removal.
	(check_event): Remove.
	(thread_db_wait): Adapt to thread events support removal.
	(thread_db_mourn_inferior): Likewise.
	(find_new_threads_callback): Likewise.
	(find_new_threads_once): Likewise.
	(thread_db_update_thread_list): Likewise.

gdb/breakpoint.h

@@ -1498,8 +1498,6 @@ extern void remove_solib_event_breakpoints (void);
    delete at next stop disposition.  */
 extern void remove_solib_event_breakpoints_at_next_stop (void);
 
-extern void remove_thread_event_breakpoints (void);
-
 extern void disable_breakpoints_in_shlibs (void);
 
 /* This function returns TRUE if ep is a catchpoint.  */