Remove "npm install loopback-explorer" log

loopback-explorer is a devDependency that is not included when
the project is installed as a dependency.

This commit changes both the explorer boot script template and `app.js`
to print the message only when the server was started via `app.listen`.

A part of https://github.com/strongloop/strong-arc/issues/380

/to @ritch or @raymondfeng please review