Remove the ops.relation module from the framework

[jnsgruk]

This PR reverses the changes introduced in #502. This may seem like a regression, though if required this functionality could be provided by a charm library. This change keeps Operator Framework more in line with its goal to behave more like the stdlib of Python charms, than a swiss-army knife.

[pengale]

Agree that this is the right direction to go here.

[niemeyer]

Thanks, this indeed looks like a wrapper to functionality easily available in the core. If helpful, a better way to start its life is indeed as a charm lib.