Remove the warning logged when sendResponse callback is being called

[rpl]

The warning (SEND_RESPONSE_DEPRECATION_WARNING) has been introduced as part of #97, along with introducing support for the sendResponse callback (which wasn't previously supported by the polyfill at all).

The warning refers to a W3C spec drafts that was created by a different W3C community group from the one being active right now, and those W3C spec drafts are now considered obsolete.

This issue is tracking the removal of the warning from the webextension-polyfill.

Some more historical context can be found in #97 (comment)