remove sandbox/linux/seccomp-legacy references from licenses.py

sandbox/linux/seccomp-legacy no longer exists in the tree, so let's remove
references to it from the license documentation generation script.

Review URL: https://codereview.chromium.org/27195007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228592 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228592