Simplify Requests::Service::Receive

In anchient times users would set up their service hooks on GitHub
manually. We then created the repository and owner instances on
our side based on the incoming request data.

This is now outdated. We require people to set the service hook on
our side, and the repository and owner records will be created at
user sync time.
request receive: validate that the repository exists
do not raise payload validation errors, just log and meter
Merge pull request #398 from travis-ci/sf-cleanup-request-receive

Simplify Requests::Service::Receive