staging: android: ion: Remove unused rbtree for ion_buffer

ion_buffer_add() insert ion_buffer into rbtree every time creating
an ion_buffer but never use it after ION reworking.
Also, buffer_lock protects only rbtree operation, remove it together.

Signed-off-by: Lecopzer Chen <lecopzer.chen@mediatek.com>
Cc: YJ Chiang <yj.chiang@mediatek.com>
Cc: Lecopzer Chen <lecopzer.chen@mediatek.com>
Acked-by: Laura Abbott <labbott@redhat.com>
Link: https://lore.kernel.org/r/20190712084717.12441-1-lecopzer.chen@mediatek.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>