[tools/perf] Remove core's implicit dependency on Telemetry

Allows to import from the core module in tools/perf without bringing
the whole of Telemetry as a dependency.

This will be particularly needed by the results_processor, to be able
to run it on a separate swarming task from where Telemetry ran.

Should be safe to land after all impacted scripts have been updated in:
https://chromium-review.googlesource.com/c/chromium/src/+/1760218

Bug: 981349
Change-Id: I75898455c3f261859ec2fe3d2cba2248c8b3ef17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1762070
Auto-Submit: Juan Antonio Navarro Pérez <perezju@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688620}

tools/perf/core/__init__.py

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
-import sys
-
-from core import path_util
-
-sys.path.append(path_util.GetTelemetryDir())