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/stackrc b/stackrc
index 2d3a599..6d8f594 100644
--- a/stackrc
+++ b/stackrc
@@ -109,9 +109,7 @@
 # Set the root URL for Horizon
 HORIZON_APACHE_ROOT="/dashboard"
 
-# Whether to use SYSTEMD to manage services, we only do this from
-# Queens forward.
-USE_SYSTEMD="True"
+# Whether to use user specific units for running services or global ones.
 USER_UNITS=$(trueorfalse False USER_UNITS)
 if [[ "$USER_UNITS" == "True" ]]; then
     SYSTEMD_DIR="$HOME/.local/share/systemd/user"