Remove obsolete OWNERS files

There are some OWNER files which don't reflect the current status.
It might be better to remove these to avoid confusion.

As far as I know, dom-team has never touched these areas.

Bug: 679390
Change-Id: Ie2d59928d8ceb44da3e75b509ee3e17ff638c6d5
Reviewed-on: https://chromium-review.googlesource.com/721104Reviewed-by: Takayoshi Kochi <kochi@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509344}

third_party/WebKit/LayoutTests/html5lib/OWNERS

-# TEAM: dom-dev@chromium.org

third_party/WebKit/LayoutTests/http/tests/xmlviewer/dumpAsText/OWNERS

-# TEAM: dom-dev@chromium.org

third_party/WebKit/LayoutTests/mathml/OWNERS

-# TEAM: dom-dev@chromium.org

third_party/WebKit/LayoutTests/xmlviewer/OWNERS

-# TEAM: dom-dev@chromium.org