staging: android: ion: Remove unused members from ion_buffer

A few members in ion_buffer struct are unused after features
like page faulting, ion_handle and ion_client were removed.
Remove these members and the leftover references to them.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Reviewed-by: Sumit Semwal <sumit.semwal@linaro.org>
Acked-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>