authPrompt: Only spin on password entries

Spinning while typing the password is a bit off-putting, and
inconsistent with how regular authentication behaves.

Only spin the spinner after typing the password.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/922

js/gdm/loginDialog.js

@@ -928,7 +928,6 @@ var LoginDialog = GObject.registerClass({
                 let answer = this._authPrompt.getAnswer();
                 this._user = this._userManager.get_user(answer);
                 this._authPrompt.clear();
-                this._authPrompt.startSpinning();
                 this._authPrompt.begin({ userName: answer });
                 this._updateCancelButton();
             });