Remove "Remove Global Feed" Crawl Optimization option

[jonoalderson]

Google's "Follow" feature (which lets you subscribe to a website via Discover) just rolled out to iOS in the US. I think that's a good indication that this is no longer an 'experiment'.

Given that this is managed via RSS, we should remove the "Disable global feed" option from our Crawl Optimization options; otherwise, people are likely to accidentally opt out of (optimal) support for the 'Follow' feature.