Bug 1584342 [wpt PR 19342] - Remove unused --channel=experimental for resources/ tests, a=testonly

Automatic update from web-platform-tests
Remove unused --channel=experimental for resources/ tests

The argument is documented as "Chrome browser channel" and isn't used
here because Chrome isn't involved.
--

wpt-commits: e2381a6676f1ae4a491cfbac5805d25d491798a2
wpt-pr: 19342

testing/web-platform/tests/.taskcluster.yml

@@ -282,7 +282,6 @@ tasks:
             export TOXENV=py27;
             ./tools/ci/run_tc.py \
               --browser=firefox \
-              --channel=experimental \
               --xvfb \
               resources_unittest \
               tools/ci/ci_resources_unittest.sh