You shouldn't delete the meta-generator tag, its used to have the stats of number of WP sites running. Though you can remove the Powered by WordPress. Matt doesn't like meta generator tag being removed. :)
Removing none of those is a violation until you remove the License that comes with WordPress or try to not give the credit to original WordPress developers in your modified WordPress if you are re-distributing it.
As @EAMann said there is nothing in license, but keeping the meta generator tag is the least you can do to thank efforts put in to WordPress Development.
UPDATE: A Ticket discussing meta generator tag on Wordpress trac.
UPDATE 2: Just for the record that Matt reference I used in my answer is because I envy Matt and I really honor him for giving away such a great software for free (both in money and in freedom). So, what ever he says about WordPress means a lot to me, you can totally ignore that if you don't feel like this. I am still looking for the link to the blog post in which Matt had an argument with a blogger regarding his advice on securing Wordpress by removing meta generator tag, will share it here as soon as I find it.