testsuite: drop "systemctl is-system-running --wait" invocation

We wait for "basic.target" being reached in the user instance anyway
before allowing the user's session to start, hence doing such a wait is
unnecessary, since that would just mean we'd wait for "default.target"
on top of "basic.target", but we shouldn#t need anything of that...

Hence, let's simplify this, reduce explicit sync points.

test/TEST-43-PRIVATEUSER-UNPRIV/testsuite.sh

@@ -10,8 +10,6 @@ runas() {
     su "$userid" -s /bin/sh -c 'XDG_RUNTIME_DIR=/run/user/$UID exec "$@"' -- sh "$@"
 }
 
-runas testuser systemctl --user --wait is-system-running
-
 runas testuser systemd-run --user --unit=test-private-users \
     -p PrivateUsers=yes -P echo hello