sys_util: remove deprecated functions
sys_util: remove deprecated functions

Remove `write` and `read` after the only user in CL:1488597 change its
usage.

TEST=cargo test -p sys_util
BUG=chromium:848187
CQ-DEPEND=CL:1488597

Change-Id: I15e03077f8970ea8350c7f64de9b64b16a08bfc1
Reviewed-on: https://chromium-review.googlesource.com/1488553
Commit-Ready: Chih-Yang Hsia <e09ac2debfefa3666e69a4e41e179a1517e8fddf@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Tested-by: Chih-Yang Hsia <e09ac2debfefa3666e69a4e41e179a1517e8fddf@chromium.org>
Reviewed-by: Chih-Yang Hsia <e09ac2debfefa3666e69a4e41e179a1517e8fddf@chromium.org>
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm