refactor: drop unmaintained ls subcommand

The correct way to handle getting the status should be ps:report.

Also cleaned up an issue where ps:report failed to catch errors for non-running containers.

tests/unit/10_ps-herokuish.bats

@@ -147,12 +147,7 @@ teardown() {
   echo "status: "$status
   assert_success
 
-  run bash -c "dokku ls"
-  echo "output: "$output
-  echo "status: "$status
-  assert_success
-
-  run bash -c "dokku --quiet ls | grep $TEST_APP | grep -q stopped"
+  run bash -c "dokku --quiet ps:report $TEST_APP | grep -q exited"
   echo "output: "$output
   echo "status: "$status
   assert_success