This reverts commit 44473361.

Reason for revert: Breaks infra repo. See crrev.com/c/2548285

Original change's description:
> Improve the setup logic for tryserver
>
> If the given change does not have a valid gerrit host name, stop
> initializing the tryserver module.
> Also polished the variable name and docstr added in crrev/c/2528670.
>
> Recipe-Nontrivial-Roll: infra
> Bug:1146487
> Change-Id: I38193b25d37c724fd41d898eb020237fbf7cecdf
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2544000
> Commit-Queue: Xinan Lin <linxinan@chromium.org>
> Reviewed-by: Nodir Turakulov <nodir@chromium.org>
> Reviewed-by: Andrii Shyshkalov <tandrii@google.com>

TBR=nodir@chromium.org,tandrii@google.com,recipe-mega-autoroller@chops-service-accounts.iam.gserviceaccount.com,linxinan@chromium.org,infra-scoped@luci-project-accounts.iam.gserviceaccount.com,sokcevic@google.com

Change-Id: I3871866454adab816ab5549f8273fe16af2d9adb
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1146487
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2548409Reviewed-by: Xinan Lin <linxinan@chromium.org>
Commit-Queue: Xinan Lin <linxinan@chromium.org>