Remove some globals from windows-nat.c

windows-nat.c has a few "count" globals that don't seem to be used.
Possibly they were used for debugging at some point, but they no
longer seem useful to me.  Because they get in the way of some code
sharing, this patch removes them.

gdb/ChangeLog
2020-04-08  Tom Tromey  <tromey@adacore.com>

	* windows-nat.c (exception_count, event_count): Remove.
	(handle_exception, get_windows_debug_event)
	(do_initial_windows_stuff): Update.