Windows gdb: Dead code in windows_nat_target::do_initial_windows_stuff

In windows_nat_target::do_initial_windows_stuff, there's no point in
setting windows_process.current_event.dwProcessId.  It's a nop, given
the following memset.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: I2fe460341b598ad293ea60d5f702b10cefc30711