Don't write to inferior_ptid in windows-nat.c, part I

The inferior_ptid hack in do_initial_win32_stuff, added back in 2008:

  https://sourceware.org/ml/gdb-patches/2008-10/msg00012.html

with:

  commit 9f9d052e
  Author:     Pierre Muller <muller@sourceware.org>
  AuthorDate: Thu Oct 2 14:20:07 2008 +0000

	      * win32-nat.c (do_initial_win32_stuff): Set inferior_ptid.

is no longer needed.  Back then, current_inferior looked like this:

  struct inferior*
  current_inferior (void)
  {
    struct inferior *inf = find_inferior_pid (ptid_get_pid (inferior_ptid));
    gdb_assert (inf);
    return inf;
  }

Nowadays, current_inferior() just returns the global current_inferior_
pointer, which didn't exist back then.

gdb/ChangeLog:
2020-06-18  Pedro Alves  <palves@redhat.com>

	* windows-nat.c (do_initial_windows_stuff): No longer set inferior_ptid.