Remove fallback matching ::cue* pseudo-element selectors

Because there is a "cue" entry in both pseudoTypeWithoutArgumentsMap and
pseudoTypeWithArgumentsMap, this path is never taken (verified with
ASSERT(0)) when running LayoutTests/media/.

It is taken for something like ::cuefoo, but that makes no sense.

This was initially the only code path:
http://trac.webkit.org/changeset/136991

Later commits made it obsolete:
http://trac.webkit.org/changeset/137955
https://src.chromium.org/viewvc/blink?revision=155834&view=revision

This was the last usage of PseudoUserAgentCustomElement, so remove that
too, together with its use counter:
https://src.chromium.org/viewvc/blink?revision=155928&view=revision
https://www.chromestatus.com/metrics/feature/timeline/popularity/103

At the time pseudos starting with "x-" were also
PseudoUserAgentCustomElement, but has since been removed:
https://src.chromium.org/viewvc/blink?revision=170210&view=revision

Review URL: https://codereview.chromium.org/646853007

git-svn-id: svn://svn.chromium.org/blink/trunk@184341 bbb929c8-8fbe-4397-9dbb-9b2b20218538