staging/android/ion: remove useless document file

After commit 9828282e ("staging: android: ion: Remove old platform
support"), the document about devicetree of ion is no need anymore, so
just remove it.

Signed-off-by: Yisheng Xie <xieyisheng1@huawei.com>
Acked-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>