Remove corsOrigin (because it is likely not necessary anymore)

Created by: sqs

@ijsnow thinks corsOrigin is no longer necessary (see https://github.com/sourcegraph/sourcegraph/pull/1954#issuecomment-455713204). Removing corsOrigin (and the need for site admins to configure it) would nicely simplify the site admin setup process. It would also be a great removal of code and tech debt.

To be specific, corsOrigin is the ONLY remaining thing in site config that site admins would need to set up in the site activation flow planned for 3.1. If we can remove corsOrigin, it would mean site admins do not need to modify site config to get activated, which would be awesome.

So if possible, please remove it ASAP so we don't need to waste time incorporating it into site admin activation flow designs.

cc @beyang for activation flow