Remove XMLHttpRequest::clearResponseBuffer().

In 2011, when clearResponseBuffer() was introduced, they chose to keep
m_response on abort() since Firefox allowed access to it.
http://trac.webkit.org/changeset/78591

Firefox now returns 0 for status, "" for statusText. getAllResponseHeaders()
returns "". Only getResponseHeader() works after abort().

The spec is all of the following methods/attributes should return emtpy/null
when error flag is set.
- getResponseHeader
- getAllResponseHeaders
- status
- statusText
- response
- responseText

We've recently fixed our implementation to conform to the spec. So, now
it's fine to clear m_response. It will never be referenced when m_error is
set. abort() sets m_error.

BUG=none

Review URL: https://codereview.chromium.org/52443002

git-svn-id: svn://svn.chromium.org/blink/trunk@160948 bbb929c8-8fbe-4397-9dbb-9b2b20218538