Don't log REGISTER anymore

It's ~25% of our logfile.

This feature was made so you can theoretically do a replay of the log, which
requires knowing which plugins were active when. But I don't think anyone's
actually doing that because it also requires other info that isn't logged.

plugins/log

@@ -43,7 +43,3 @@ sub hook_user_balance($class, $user, $old, $delta, $new, $transaction_id, @) {
 sub hook_checkout($class, $cart, $username, $transaction_id, @) {
     _log("CHECKOUT $transaction_id $_") for map $_->as_loggable, $cart->entries;
 }
-
-sub hook_register($class, $plugin, @) {
-    _log("REGISTER $plugin");
-}