staging: android: ion: Remove unused rbtree for ion_buffer

Re-apply commit 3e6998b1 ("staging: android: ion: Remove unused
rbtree for ion_buffer") which has been inadvertently and partly reverted
by merge commit bfa0399b
("Merge Linus's 5.4-rc1-prerelease branch into android-mainline").
Later, the following two commits completed the incomplete revert:

commit c8e152bc ("ANDROID: staging: ion: Fix missing entry in
kerneldoc header")
commit f9aa2c60 ("ANDROID: ion: Fix buffer_lock mutex
initialization")

This commit re-applies the original commit, i.e. it removes the unused
rbtree again.

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.

Change-Id: I56cc75fbc84d7412fb4d090f577b5d4a6980b879
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>
(cherry picked from commit 524fcca39b72bf4f6a32fcbf051dd0a8b0e67634)