Drop slower Node::hasTagName(QualifiedName) overload

Drop slower Node::hasTagName(QualifiedName) overload. It is only used by
SiblingTraversalStrategies at this point. The objective is to encourage
future callers to use the faster alternatives:
- HTMLElement::hasTagName(HTMLQualifiedName) / SVGElement::hasTagName(SVGQualifiedName)
- Node::hasTagName(HTMLQualifiedName) / Node::hasTagName(SVGQualifiedName)
- Element::hasTagName(QualifiedName)

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/415823002

git-svn-id: svn://svn.chromium.org/blink/trunk@179165 bbb929c8-8fbe-4397-9dbb-9b2b20218538