Remove BrightIdSponsor contract

[yuetloo]

This issue tracks the unfinished work in #748:

1. Remove the BrightIdSponsor contract
2. Update the BrightIdUserRegistry contract to remove references to the BrightIdSponsor contract.

The BrightId app will automatically do the sponsorship request. Therefore, there's no need to make the request from the clr.fund app.