If using Python, you can delete an index by first removing the entry in your index.yaml file, and then updating your site. Then you must issue this command:
appcfg.py vacuum_indexes myapp/
With Django (what I am using), you use this command:
What is nice about the process is that, for each index that has been removed from your index.yaml, it will confirm whether you want to delete it or not. So you get a second chance to make sure you are right. If you delete the wrong index, it’s going to take some time for it to rebuild, and portions of your site could be down.
Why do I need this:
I don’t think you would see a noticeable speed improvement to your data inserts, updates, or deletes by having a few less unused indexes, but it couldn’t hurt. Also you are limited to 100 indexes (as of April 2010) so this how you free them up.