Revert "update_chroot: Also update target-sdk-post-cross"

This reverts commit 8e9b893bf4e11cfb9df3e68c0da22fe9982d7f6c.

Reason for revert: This is breaking chromiumos-sdk builder.

More details:
Building rust needs cross-* packages to be available but that happens
 very late in SDKBuildToolChains stage in SDK builder.
This is why a special target-sdk-post-cross package was specially crafted.

It CANNOT be built as same time as target-sdk.

Breakage tryjob link:
https://cros-goldeneye.corp.google.com/chromeos/healthmonitoring/buildDetails?buildbucketId=8949410578903822208

Original change's description:
> update_chroot: Also update target-sdk-post-cross
>
> Add virtual/target-sdk-post-cross to the list of packages updated by
> update_chroot.  This ensures that packages like rust and cargo are
> properly updated.
>
> BUG=chromium:820330
> TEST=./update_chroot updates rust to 1.24
>
> Change-Id: I5604b91a57b6a3ca4207e6cb1d6dccb6b6d45ad0
> Signed-off-by: Chirantan Ekbote <chirantan@chromium.org>
> Reviewed-on: https://chromium-review.googlesource.com/957936
> Reviewed-by: Zach Reizner <zachr@chromium.org>

Bug: chromium:820330
Change-Id: I124990f79ac1e053c11206c6a97b7e25d921b3ca
Reviewed-on: https://chromium-review.googlesource.com/1011737
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Commit-Queue: Manoj Gupta <manojgupta@chromium.org>
1 file changed