Remove leftover references to files/pips/

The files/pips/* files were removed a while ago (replaced by
pip-requires in individual projects). So remove leftover code that was
dealing with that.

Change-Id: Id521a3365ab018193607389f022a25acddb49714
diff --git a/tools/build_uec.sh b/tools/build_uec.sh
index 48819c9..58c5425 100755
--- a/tools/build_uec.sh
+++ b/tools/build_uec.sh
@@ -68,7 +68,7 @@
 # Option to warm the base image with software requirements.
 if [ $WARM_CACHE ]; then
     cd $TOOLS_DIR
-    ./warm_apts_and_pips_for_uec.sh $image_dir/disk
+    ./warm_apts_for_uec.sh $image_dir/disk
 fi
 
 # Name of our instance, used by libvirt