This property isn't used anymore.

Bug: 891917
Change-Id: I005f5b320cb33afddb1d3272f3f626542996bb62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3766100Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Joanna Wang <jojwang@chromium.org>