Revert "Change telemetry_unittests to use python3"

This reverts commit 39a37fa.

Reason for revert: Suspected cause of crbug.com/1260124

Original change's description:
> Change telemetry_unittests to use python3
>
> Bug: 1252120
> Change-Id: I2c81aacf7f3e6366412dedb6a13334744a5f21a9
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3193889
> Reviewed-by: Wenbin Zhang <wenbinzhang@google.com>
> Reviewed-by: Dirk Pranke <dpranke@google.com>
> Commit-Queue: Yuan Huang <yuanhuang@google.com>
> Cr-Commit-Position: refs/heads/main@{#931640}

Bug: 1252120
Change-Id: I1eeeaf887c80306756674c61be6fb39e3e2645d6
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3224363
Owners-Override: John Williams <jrw@chromium.org>
Owners-Override: Nic Hollingum <hollingum@google.com>
Commit-Queue: Nic Hollingum <hollingum@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Nic Hollingum <hollingum@google.com>
Cr-Commit-Position: refs/heads/main@{#931871}

testing/buildbot/gn_isolate_map.pyl

@@ -1767,7 +1767,6 @@
     "label": "//chrome/test:telemetry_unittests",
     "script": "//testing/scripts/run_telemetry_as_googletest.py",
     "type": "script",
-    "python3": True,
   },
   "telemetry_weblayer_apks": {
     "label": "//tools/perf/chrome_telemetry_build:telemetry_weblayer_apks",