chore: Remove unused App component breaking test watch mode

- Updated Jest dep to `^23.4.0`, fixing jestjs/jest#6407 which caused our `test:dev` script to appear to hang when files were modified and Jest cache was absent. The original behavior froze at this message:

       Determining test suites to run...

  The new version of Jest properly prints the error message:

       Determining test suites to run...

       Configuration error:

       Could not locate module src/store/reducers/navigation mapped as:
       /Users/jzetlen/gits/pwa/pwa-studio/packages/venia-concept/src/store/reducers/navigation.

       Please check your configuration for these entries:
       {
         "moduleNameMapper": {
           "/^src\/(.+)/": "/Users/jzetlen/gits/pwa/pwa-studio/packages/venia-concept/src/$1"
         },
         "resolver": null
       }

- Removed the `App` component, which is now unused due to the `MagentoResolver` calling `RootComponents` directly, and was the source of the mapping error above. Future errors of this kind will appear early and should be easier to diagnose and fix.