"clear_username_email_history" upgrade script is not compatible with latest master

[julianlam]

Not sure when it broke, but ran into this today with an old database.

The error states that callback is undefined in line 41:

NodeBB/src/upgrades/1.12.1/clear_username_email_history.js

Lines 40 to 42 in 2adae87

	function (err) {
	callback(err);
	});

... Probably because callback is no longer passed in if it is an asynchronous function.