Remove redundant/misleaing README.chromium files.

These files are a legacy from when the native_client_sdk
and ppapi were DEPS'd in from an external repos rather
than being part of chromium.

R=binji@chromium.org, brettw@chromium.org

Review URL: https://codereview.chromium.org/54983002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232428 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232428