Remove cleanups from ser-mingw.c

This removes the only cleanup from ser-mingw.c, replacing it with a
specialization of std::unique_ptr.

Tested by the buildbot.

gdb/ChangeLog
2018-06-18  Tom Tromey  <tom@tromey.com>

	* ser-mingw.c (struct pipe_state_destroyer): New.
	(pipe_state_up): New typedef.
	(cleanup_pipe_state): Remove.
	(pipe_windows_open): Use pipe_state_up.  Don't release argv.