Revert "Meteor.loginWith<External> now waits for config"

This reverts commit 76ded8feb27a0d83f0b1857aa07e3a7c3d00c7d3.
Improve error message for pre-config login

Addresses #2048.

An earlier attempt (to wait for the config to load) ran into popup
blockers.

It would be nice to load the config statically with something like
Arunoda's fast-render. That said, even that's not good enough to allow
OAuth logins that bypass the popup blocker that aren't a result of a
user action, and for user actions it's easy enough to gate your login
button on `Accounts.loginServicesConfigured()`.

Longer term solutions include non-popup methods of OAuth login (see
Issue #438).