remove additional f18 references

f18 has been EOL for 6 weeks now, time to purge it from devstack

Change-Id: I5aac2c63b2f4cd8b01ae685b1acf4c188637558b
diff --git a/files/rpms/cinder b/files/rpms/cinder
index 199ae10..423d57c 100644
--- a/files/rpms/cinder
+++ b/files/rpms/cinder
@@ -4,4 +4,4 @@
 python-devel
 postgresql-devel
 iscsi-initiator-utils
-python-lxml         #dist:f18,f19,f20,rhel7
+python-lxml         #dist:f19,f20,rhel7