Remove WTF::textAtom and WTF::commentAtom

These are only used in implementations of Node::nodeName(), where
CDATASection, Document and DocumentFragment already use raw strings.

Added in <https://trac.webkit.org/changeset/9639>, at which point both
atoms were used in multiple places. Previously raw strings were used.

BUG=none

Review URL: https://codereview.chromium.org/106483002

git-svn-id: svn://svn.chromium.org/blink/trunk@163281 bbb929c8-8fbe-4397-9dbb-9b2b20218538