(Windows) remove thread notification for main thread of inferior

(Windows) remove thread notification for main thread of inferior

This is a followup on a recent patch which, among other things
introduced the exit notification of the main thread in order
to be symetrical with the fact that a thread notification was
emitted before signaling its creation.

This patch takes the opposite approach of removing both creation
and exit notifications for that main thread, which is consistent
with what is done on other platforms such as GNU/Linux for instance.

gdb/ChangeLog

	* windows-nat.c (windows_add_thread): Add new parameter
	"main_thread_p" with default value set to false.  Update
	function documentation as well as all callers.
	(windows_delete_thread): Likewise.
	(fake_create_process): Update call to windows_add_thread.
	(get_windows_debug_event) <CREATE_THREAD_DEBUG_EVENT>
	<CREATE_PROCESS_DEBUG_EVENT>: Likewise.
	<EXIT_THREAD_DEBUG_EVENT, EXIT_PROCESS_DEBUG_EVENT>: Update
	call to windows_delete_thread.

Tested on x86-windows (MinGW) using AdaCore's testsuite.