debugfs: remove rmdir() non-empty complaint

Hi,

This patch kills the pointless debugfs rmdir() printk() when called on a
non-empty directory. blktrace will sometimes have to call it a few times
when forcefully ending a trace, which polutes the log with pointless
warnings.

Rationale:

- It's more code to work-around this "problem" in the debugfs users, and
  you would have to add code to check for empty directories to do so (or
  assume that debugfs is using simple_ helpers, but that would be a
  layering violation).

- Other rmdir() implementations don't complain about something this
  silly.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>