gerrit: set Content-Type: application/json for deleting request.
gerrit: set Content-Type: application/json for deleting request.

Gerrit call get unexpected HTTP 400 response:
https://screenshot.googleplex.com/eDwh8steCrg

It's probably because
https://gerrit-review.googlesource.com/Documentation/rest-api.html#_400_bad_request,
json input is expected but we pass in others.

Switch to use call() to force input content of header is application/json.

BUG=chromium:1008442
TEST=None

Change-Id: I090eed9a897742bc7a6e9fe541bb71fb5e0c7327
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1846774
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go