rollback: Remove unused rollback_lock function
rollback: Remove unused rollback_lock function

Use of the rollback_lock function was removed in
https://crrev.com/c/479176.

BRANCH=none
BUG=none
TEST=make buildall -j

Change-Id: I15bfeba9b169c7b0fae8d3c9423bc2f4817d52d8
Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1902460
Reviewed-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec