[2.0] Remove loopback-explorer from dev deps

 1. loopback-explorer has a peer dependency on loopback, which
  forces `npm install` to install loopback within loopback. Somehow
  npm ends up installing loopback 1.x, which is not compatible with
  datasource-juggler 2.x.

 2. The only place using loopback-explorer is the app used by e2e tests.
  However, the e2e test app does not load the explorer at the moment
  and the e2e test are not being run anyway.

/to @ritch 

In the longer term, I'd like to refactor loopback-explorer to check loopback version at runtime, using the `app` object provided to the middleware factory.