Update setUseFormatCookie() to use WebResponse::setcookie()

We previously relied on PHP's setcookie function rather than
WebResponse::setcookie() due to some nuances of how MW's version of
setcookie behaved which was causing problems with setUseFormatCookie()
on the WMF cluster (iirc, it was around wgCookieHttpOnly). But
WebResponse::setcookie() has since been updated to support more flexible
options, meaning we can now rely on it again.

Change-Id: I5c173b4f8c079bedefc57b1c45982c1576538b0e