Revert "introduce $SYSTEMD_SYSTEM_UNIT_DIR"

This reverts commit 4bf38a829a00f1866e6d93f50d784694e16e93c2.

This variable is pointless since we can always replace it with
/lib/systemd.

Commit I459f7514ab35082d31607968252a9005fa25de2a will fix the issue
in tripleo-image-elements.

Change-Id: I524badb6836a0d04a5f6e6a5b5d95920fd923ef6
Merge "Revert "introduce $SYSTEMD_SYSTEM_UNIT_DIR""