This reverts commit e79ddeaa.

Reason for revert: https://bugs.chromium.org/p/chromium/issues/detail?id=748486

Original change's description:
> gclient flatten: preserve variable placeholders
> 
> One of the main use cases is making it clear which revision hashes
> need to be changed together. The way it's usually done is one variable
> referenced several times. With this CL, we preserve the references
> from original DEPS, as opposed to evaluating them and losing some info.
> 
> This CL actually makes Var() emit a variable placeholder
> instead of its value, and adds support for these placeholders
> to gclient.
> 
> One of possible next steps might be to deprecate Var().
> 
> Bug: 570091
> Change-Id: I9b13a691b5203cc284c33a59438720e31c9ebf7a
> Reviewed-on: https://chromium-review.googlesource.com/583617
> Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>

TBR=phajdan.jr@chromium.org,dpranke@chromium.org

Change-Id: If9c52ebfa78aba8041ce797ff842d09952d0e2ce
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 570091, 748486
Reviewed-on: https://chromium-review.googlesource.com/584907Reviewed-by: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>