Remove outdated FIXME comment from Document::decrementActiveParserCount()

Remove outdated FIXME comment from Document::decrementActiveParserCount(). This
comment was added by Eric in http://trac.webkit.org/changeset/143753 and was
associated to the #if ENABLE(THREADED_HTML_PARSER) check. However, those checks
were removed in Blink at:
https://src.chromium.org/viewvc/blink?revision=148736&view=revision

As a result, the comment no longer makes sense as it is already "always
enabled". The same comment in DocumentLoader.cpp was correctly removed with the
THREADED_HTML_PARSER defined, only the one in Document.cpp remained.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/366703005

git-svn-id: svn://svn.chromium.org/blink/trunk@177339 bbb929c8-8fbe-4397-9dbb-9b2b20218538