Don't write to inferior_ptid in procfs.c

The inferior_ptid write in procfs_do_thread_registers should be
unnecessary because the target fetch_registers method should (and
does) extract the ptid from the regcache.

Not tested.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	* procfs.c (procfs_target::attach): Don't write to inferior_ptid.
	(procfs_target::detach): Use switch_to_no_thread
	instead of writing to inferior_ptid directly.
	(do_attach): Change return type to void.  Switch to the added
	thread.
	(procfs_target::create_inferior): Switch to the added thread.
	(procfs_do_thread_registers): Don't write to inferior_ptid.