Revert "introduce $SYSTEMD_SYSTEM_UNIT_DIR"

This reverts commit 4bf38a829a.

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

elements/base/environment.d/10-SYSTEMD_SYSTEM_UNIT_DIR.bash

@@ -1,8 +0,0 @@
-SYSTEMD_PC_FILE="/usr/share/pkgconfig/systemd.pc"
-SYSTEMD_SYSTEM_UNIT_DIR="/lib"
-if [ -f $SYSTEMD_PC_FILE ]; then
-    SYSTEMD_SYSTEM_UNIT_DIR=$(awk \
-        '/systemdsystemunitdir=/ {split($0,a,"=");print a[2]}' \
-        $SYSTEMD_PC_FILE)
-fi
-export SYSTEMD_SYSTEM_UNIT_DIR