This reverts commit 529d6a4e.

Reason for revert: broke developers and CI/Try checkouts.

Bug: 735418

Original change's description:
> gclient: include deps_os entries in dependencies
> 
> Keep deps_os entries in dependencies, just with should_process set to False
> for entries not applicable to target OS list.
> 
> This way gclient flatten has proper access to dependencies e.g. to evaluate
> recursedeps referring to deps_os entries other than active OS.
> 
> Allow but ignore deps_os overriding a value with None, since that does not
> fit the new model. There's no correctness harm in not checking out a repo.
> 
> Allow "overrides" setting given dependency to the same value. This seems
> fairly common, especially for mac/ios and unix/android, even in chromium/src.
> 
> Bug: 570091
> Change-Id: I2037a1ecc5fd2da6b5f73061548b81fc79ba2e72
> Reviewed-on: https://chromium-review.googlesource.com/541280
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>

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

Change-Id: Iaa0c39865908a5b25c15dda54ba61c0e76abcbea
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 570091
Reviewed-on: https://chromium-review.googlesource.com/543138Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>