The linux command find has a very handy option -exec which lets you execute any linux command on the files returned by a find operation. Think of it as pipping|if you may.

cd /path/to/folder/with/svn/folders
find . -name ".svn" -type d -exec rm -rf {} \;

That should do it. Also next time just do a svn export instead of an svn checkout, that’ll leave all the .svn stuff out.