Remove some superfluous license files

- lcov - LICENCE is identical to LICENSE and seems to have been added
  accidentally in http://codereview.chromium.org/57083.
- libxml - COPYING is identical to Copyright and appears not to exist upstream
  at http://git.gnome.org/browse/libxml2/tree.
- libxml - COPYING is identical to Copyright and appears not to exist upstream
  at http://git.gnome.org/browse/libxslt/tree.

BUG=138921


Review URL: https://chromiumcodereview.appspot.com/10834110

git-svn-id: http://src.chromium.org/svn/trunk/src/third_party/lcov@149720 4ff67af0-8c30-449e-8e8b-ad334ec8d88c