This reverts commit 55e5853e.

Reason for revert:

  File "PRESUBMIT.py", line 11, in CheckChangeOnCommit
    return input_api.canned_checks.CheckChangedLUCIConfigs(input_api, output_api)
  File "/usr/local/google/home/abennetts/cr/depot_tools/presubmit_canned_checks.py", line 1421, in CheckChangedLUCIConfigs
    acc_tkn = authenticator.get_access_token()
  File "/usr/local/google/home/abennetts/cr/depot_tools/auth.py", line 414, in get_access_token
    if not self._external_token and allow_user_interaction:
AttributeError: 'Authenticator' object has no attribute '_external_token'


Original change's description:
> depot_tools: Stop using oauth2client
> 
> Bug: 1001756
> Change-Id: I8a0ca2b0f44b20564a9d3192543a7a69788d8d87
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1854898
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
> Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>

TBR=vadimsh@chromium.org,ehmaldonado@chromium.org

Change-Id: I94cf38e82e53e51c66efcb99c51f0e1418e86f49
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1001756, 1015285
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1866184Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>