remove 'chromium_builder_tests' target from GN

It existed only for compatibility with GYP and now all the required
pieces have been updated, and thus it can now be removed.

BUG=None

Change-Id: I66eff47aaeebae17e795892208bf04ce1d1cd803
Reviewed-on: https://chromium-review.googlesource.com/558405
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Thiago Farina <tfarina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#483844}
4 files changed