(core) Remove LoginSession, which was mainly serving situations that are no longer used.

Summary:
In the past, Cognito sign-ins were intended to give authorization to some AWS
services (like SQS); various tokens were stored in the session for this
purpose. This is no longer used. Profiles from Cognito now serve a limited
purpose: first-time initialization of name and picture, and keeping track of
which login method was used. For these remaining needs, ScopedSession is
sufficient.

Test Plan:
Existing test pass. Tested manually that logins work with Google and
Email + Password. Tested manually that on a clean database, name and picture
are picked up from a Google Login.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2907

package.json

@@ -52,7 +52,7 @@
     "chai": "4.2.0",
     "chai-as-promised": "7.1.1",
     "mocha": "5.2.0",
-    "mocha-webdriver": "0.2.8",
+    "mocha-webdriver": "0.2.9",
     "moment-locales-webpack-plugin": "^1.2.0",
     "nodemon": "^2.0.4",
     "selenium-webdriver": "3.6.0",