Revert "Add POST support to XHR as well as .status and statusText support"

This reverts commit 5ef81d249b841f44c9593ffb0158d64c7e0febfc.

This appeared to make 5 XHR tests timeout.  Not sure why yet.
TBR=esprehn@chromium.org

Review URL: https://codereview.chromium.org/808663002