This reverts commit d328b47e.

Reason for revert: suspect this caused https://bugs.chromium.org/p/chromium/issues/detail?id=863211

Original change's description:
> gclient: Add support for the branch:revision format.
> 
> Bug: 850812, 853032
> Change-Id: I597acbde2b3c229813b7eba8fcba52d5877130b2
> Reviewed-on: https://chromium-review.googlesource.com/1119235
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
> Reviewed-by: Aaron Gable <agable@chromium.org>

TBR=agable@chromium.org,tandrii@chromium.org,ehmaldonado@chromium.org

Change-Id: I1e4c00b83a2840cc5a87621469da866dad58a20c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 850812, 853032
Reviewed-on: https://chromium-review.googlesource.com/1135893Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>