staging: android: ion: Clean unused debug_show memeber of the heap object

ION had supported heap debug info under /sys/kernel/debug/ion/<heap_name>.
This support have been removed but some leftovers (dead code) still exist.

This patch removes the existing dead code.

Fixes: 15c6098c

 ("staging: android: ion: Remove ion_handle and ion_client")
Signed-off-by: Alexey Skidanov <alexey.skidanov@intel.com>
Acked-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>