Remove all proposal-related code

We're not using proposals at the moment, so it's just in the way.

tests/unit/models/proposal-test.js

@@ -1,8 +0,0 @@
-// import { moduleFor, test } from 'ember-qunit';
-//
-// moduleFor('model:proposal', 'Unit | Model | proposal');
-//
-// test('contributor relation', function(assert) {
-//   // TODO: Test contributor relation
-//   assert.ok(true);
-// });