drivers: habanalabs: remove unused dentry pointer for debugfs files

The dentry for the created debugfs file was being saved, but never used
anywhere.  As the pointer isn't needed for anything, and the debugfs
files are being properly removed by removing the parent directory,
remove the saved pointer as well, saving a tiny bit of memory and logic.

Cc: Oded Gabbay <ogabbay@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Tomer Tayar <ttayar@habana.ai>
Cc: Moti Haimovski <mhaimovski@habana.ai>
Cc: Omer Shpigelman <oshpigelman@habana.ai>
Cc: Ofir Bitton <obitton@habana.ai>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>