create_test_jar.sh: Clean out tmp directories.

This script creates tmp directories /tmp/delegate_last_* where it
creates some files before copying them to their final destination.
This CL adds commands to remove each of those tmp directories
immediately before it is created, to ensure that nothing remains in
them from a previous run.

The directories are NOT cleaned up at the end since they could be
useful for debugging the script.

Test: Ran create_test_jar.sh more than once in a row.

Change-Id: I5acaaa4f7b17155f769c68770cde5af02dd58ff4