Don't write to inferior_ptid in fork-child.c

This is no longer necessary.  All targets that call fork_inferior now
also call switch_to_thread as soon as they add the main thread.

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

	* fork-child.c (postfork_hook): Don't write to inferior_ptid.