Stop forcing trace level 2 on the AWS suite

The diagnostic setenv has served its purpose: the AWS suite's runtime was
an unscoped describe_images scan amplified by a tiny transport page size,
and both are being fixed at source. Leaving level 2 on would keep emitting
one log line per boto invocation - roughly 12,500 lines a run - for a
question that is now answered.

CB_TEST_TRACE stays in passenv, so the tracing remains available on demand
for the next time a cloud suite needs explaining; it is just no longer on
by default.

tox.ini

@@ -28,15 +28,6 @@ setenv =
     # Fix for moto import issue: https://github.com/travis-ci/travis-ci/issues/7940
     BOTO_CONFIG=/dev/null
     aws: CB_TEST_PROVIDER=aws
-    # DIAGNOSTIC - REMOVE BEFORE MERGING. Turns on the wait/retry tracing
-    # described in tests/conftest.py for the AWS suite. It lives here rather
-    # than in the workflow env because the cloud workflow triggers on
-    # pull_request_target, so its definition comes from the base branch and a
-    # PR cannot change it; tox.ini is checked out from the PR head and can.
-    # Level 2 also logs each boto invocation. Level 1 already showed the AWS
-    # image test spends 1184 of its 1400 seconds in one stretch with no
-    # polling and no retries, so the remaining question is which call blocks.
-    aws: CB_TEST_TRACE=2
     azure: CB_TEST_PROVIDER=azure
     gcp: CB_TEST_PROVIDER=gcp
     openstack: CB_TEST_PROVIDER=openstack