mttools: Remove usage of deprecated Json::Reader.

Upgrading jsoncpp to upstream has revealed that Json::Reader has been
deprecated in favour of CharReader and CharReaderBuilder.

BUG=b:156443138
TEST=FEATURES="test" emerge-octopus chromeos-base/mttools

Change-Id: I18a751843f865a97ede4e011241c783760efebb5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/mttools/+/2203248
Tested-by: Stuart Langley <slangley@chromium.org>
Commit-Queue: Stuart Langley <slangley@chromium.org>
Reviewed-by: Sean O'Brien <seobrien@chromium.org>
1 file changed