mirage: Remove default scenario

Supporting mirage in development builds is difficult without `ember-cli-mirage` since without proper tree shaking we would be shipping the additional mirage dependency bytes to production as well. Since most development happens in `start:live` mode and with help of the test suite, the use of this default scenario is somewhat questionable anyway.

Author: Turbo87

docs/CONTRIBUTING.md

@@ -138,15 +138,9 @@ talk to:
 | ----------------------------------------- | --------------------------------------------- | ------------------------------------------------------- |
 | `pnpm start:live`                         | <https://crates.io>                           | Testing UI changes with the full live site's data       |
 | `pnpm start:staging`                      | <https://staging-crates-io.herokuapp.com>     | Testing UI changes with a smaller set of realistic data |
-| `pnpm start`                              | Static fixture test data in `mirage/fixtures` | Setting up particular situations, see note              |
 | `pnpm start:local`                        | Backend server running locally                | See the Working on the backend section for setup        |
 | `pnpm start -- --proxy https://crates.io` | Whatever is specified in `--proxy` arg        | If your use case is not covered here                    |
 
-> Note: If you want to set up a particular situation, you can edit the fixture
-> data used for tests in `mirage/fixtures`. The fixture data does not currently
-> contain JSON needed to support every page, so some pages might not load
-> correctly.
-
 #### Running the frontend tests
 
 You can run the frontend tests with:

mirage/scenarios/default.js

@@ -24,7 +24,6 @@
     "precompress": "node ./script/precompress-assets.mjs",
     "prettier:check": "prettier --check package.json '**/*.js' '**/*.mjs' '**/*.ts' '**/*.md'",
     "prettier:write": "prettier --write package.json '**/*.js' '**/*.mjs' '**/*.ts' '**/*.md'",
-    "start": "ember serve",
     "start:docker": "ember serve --proxy http://backend:8888",
     "start:live": "ember serve --proxy https://crates.io",
     "start:local": "ember serve --proxy http://127.0.0.1:8888",