Remove opencensus dep

[Stebalien]

Unfortunately, this means that anything wanting to use the libp2p interfaces (even light-clients that leave all the heavy lifting up to a daemon) need to import opencensus.

From the looks of it, we aren't actually using any of the code in metrics/register.go. We might want to consider removing this code unless we think we're actually going to use it.

Note: this pulls in stuff like grpc, google's cloud API libraries, etc.