tee: remove linked list of struct tee_shm

mainline inclusion
from mainline-v5.7-rc1
commit 59a135f6
category: cleanup
bugzilla: NA
CVE: NA

Prepare for fixing CVE-2021-44733.
--------------------------------

Removes list_shm from struct tee_context since the linked list isn't used
any longer.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Conflicts:
  drivers/tee/tee_core.c
[yyl: adjust context]
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>