Bug 1642545 - run mach taskcluster-build-image with python 3 r=gbrown

Changes:

No changes were required to run taskcluster-build-image with python3.5.

Differential Revision: https://phabricator.services.mozilla.com/D77755

mach

@@ -42,7 +42,6 @@ py2commands="
     rusttests
     static-analysis
     talos-test
-    taskcluster-build-image
     taskcluster-load-image
     taskgraph
     telemetry-tests-client