toolchain: drop InsidePrepareForBuild endpoint.

The endpoint is both unused and unnecessary.  If the PrepareForBuild
endpoint needs to enter the chroot, it can.

BUG=chromium:1019868
TEST=generate.sh

Change-Id: Ib3878e13b0986f1c82eef7cc21a3ef70172d5e39
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/1993659
Reviewed-by: David Burger <dburger@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
Commit-Queue: LaMont Jones <lamontjones@chromium.org>
Tested-by: LaMont Jones <lamontjones@chromium.org>
3 files changed