Clean up remainders of USE_SYSTEMD and logging setup

We dropped the ability to override USE_SYSTEMD to False when we deleted
screen support in [0], so we can also clean up any conditionals based
on it.

Also clean up the logging setup functions, dropping local vars for
parameters that we don't actually support anymore.

[0] I8c27182f60b0f5310b3a8bf5feb02beb7ffbb26a

Change-Id: I5cbce9f2c42e111761e8689447b3f8cbb7ea2eb5
diff --git a/clean.sh b/clean.sh
index d6c6b40..685a719 100755
--- a/clean.sh
+++ b/clean.sh
@@ -123,12 +123,10 @@
     sudo rm -rf $LOGDIR
 fi
 
-# Clean out the systemd user unit files if systemd was used.
-if [[ "$USE_SYSTEMD" = "True" ]]; then
-    sudo find $SYSTEMD_DIR -type f -name '*devstack@*service' -delete
-    # Make systemd aware of the deletion.
-    $SYSTEMCTL daemon-reload
-fi
+# Clean out the systemd unit files.
+sudo find $SYSTEMD_DIR -type f -name '*devstack@*service' -delete
+# Make systemd aware of the deletion.
+$SYSTEMCTL daemon-reload
 
 # Clean up venvs
 DIRS_TO_CLEAN="$WHEELHOUSE ${PROJECT_VENV[@]} .config/openstack"