Bug 1926291 - clean up ubuntu 16.04 scars from test scripts. r=taskgraph-reviewers,bhearsum

Differential Revision: https://phabricator.services.mozilla.com/D226487

taskcluster/gecko_taskgraph/transforms/job/mozharness_test.py

@@ -156,10 +156,6 @@ def mozharness_test_on_docker(config, job, taskdesc):
 
     env["PYTHON"] = "python3"
 
-    # Legacy linux64 tests rely on compiz.
-    if test.get("docker-image", {}).get("in-tree") == "desktop1604-test":
-        env.update({"NEED_COMPIZ": "true"})
-
     # Bug 1602701/1601828 - use compiz on ubuntu1804 due to GTK asynchiness
     # when manipulating windows.
     if test.get("docker-image", {}).get("in-tree") == "ubuntu1804-test":

taskcluster/scripts/tester/test-linux.sh

@@ -166,9 +166,7 @@ fi
 if $NEED_WINDOW_MANAGER; then
     # This is read by xsession to select the window manager
     . /etc/lsb-release
-    if [ $DISTRIBUTION == "Ubuntu" ] && [ $RELEASE == "16.04" ]; then
-        echo DESKTOP_SESSION=ubuntu > $HOME/.xsessionrc
-    elif [ $DISTRIBUTION == "Ubuntu" ] && [ $RELEASE == "18.04" ]; then
+    if [ $DISTRIBUTION == "Ubuntu" ] && [ $RELEASE == "18.04" ]; then
         echo export DESKTOP_SESSION=gnome > $HOME/.xsessionrc
         echo export XDG_CURRENT_DESKTOP=GNOME > $HOME/.xsessionrc
         if [ $MOZ_ENABLE_WAYLAND ]; then