This reverts commit 8b64ddec.

Reason for revert: it appears we need this, since .DEPS.git
specifies custom dependencies needed for tryjob.

Original change's description:
> Remove unused functionality in bot_update.py
>
> .DEPS.git has been removed in 2014 and no longer supported. It was
> deleted in https://codereview.chromium.org/497313003 (August 2014). Last
> entry in chromium/src is from tag 44.0.2403.47 in June 2015.
>
> Recipe-Nontrivial-Roll: build
> Recipe-Nontrivial-Roll: build_limited
> Recipe-Nontrivial-Roll: chrome_release
> Recipe-Nontrivial-Roll: chromiumos
> Recipe-Nontrivial-Roll: infra
> Change-Id: I3ffb8813af867df959098c02d399c8d745894aae
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3439573
> Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
> Commit-Queue: Josip Sokcevic <sokcevic@google.com>

Change-Id: I36bcd352a984b4b762096197a384e86347418290
Recipe-Nontrivial-Roll: build
Recipe-Nontrivial-Roll: build_limited
Recipe-Nontrivial-Roll: chrome_release
Recipe-Nontrivial-Roll: chromiumos
Recipe-Nontrivial-Roll: infra
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3448707
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>