Neuter meek-http-helper's default proxy setting

The headless meek-http-helper browser undoes Tor Browser's proxy setting: https://gitweb.torproject.org/builders/tor-browser-bundle.git/blob/5400da654020a34edb9edee70a0583a89231c4fe:/Bundle-Data/PTConfigs/meek-http-helper-user.js#l7 {{{ // 0 is "No proxy". user_pref("network.proxy.type", 0); }}} This setting used to be necessary in order for the HTTPS requests to be made on the network without themselves trying to go through the local tor proxy. However, since legacy/trac#12120 (moved), we set the proxy type individually for every request (including a "direct" non-proxy when TOR_PT_PROXY is unset), so it's no longer necessary to change the global setting.

A good reason to leave the proxy set is so if someone manages to start Firefox using the meek-http-helper profile as a normal non-headless browser, it should fail closed, and give "the proxy server is refusing connections" rather than acting as an unproxied browser.

Even better, we can set the proxy URL to 127.0.0.1:9, the discard port, so it will fail even closeder if tor happens to be running on the usual port set by Tor Browser.