cleanup plugin resources

git-svn-id: https://plugins.svn.wordpress.org/akismet/trunk@873047 b8457f37-d9ea-0310-8a92-e5e31aec5664
drop support for legacy installs

git-svn-id: https://plugins.svn.wordpress.org/akismet/trunk@873049 b8457f37-d9ea-0310-8a92-e5e31aec5664