Fixed eventsource/format-utf-8.htm

https://bugzilla.mozilla.org/show_bug.cgi?id=869878

As discussion in the above bug, an event stream's
(http://www.w3.org/TR/eventsource/#text-event-stream)
charset parameter serves no purpose; it is only allowed
for compatibility with legacy servers.

Moreover, since the spec says nothing about "the charset
parameter is present and the value is not utf-8", this
fixing just comments out the assert_unreached() called
by onmessage().

Signed-off-by: wanmingx.lin <wanmingx.lin@intel.com>