toolchain_utils: Use external chrome source tree for telemetry.

When generating the nightly performance tests, specify an external
Chrome source tree, since Chrome sync'ing is broken inside
Chrome OS.  Also remove unnecessary code that references a file that
no longer exists.

BUG=chromium:846508
TEST=Successfully ran nightly tests with these changes on Chrotomation2.

Change-Id: I1cba94f82ad6ffb39c941a5dc3cb8f95cb833d20
Reviewed-on: https://chromium-review.googlesource.com/1072870


Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Caroline Tice <cmtice@chromium.org>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>