chore(cleanup): Remove signer-stub.js (#1541), r=@seanmonstar
chore(cleanup): Remove signer-stub.js (#1541), r=@seanmonstar


mozilla/fxa-auth-server,eoger/fxa-auth-server,mozilla/fxa-auth-server,eoger/fxa-auth-server,mozilla/fxa-auth-server,eoger/fxa-auth-server,eoger/fxa-auth-server,mozilla/fxa-auth-server