Remove mention of proto_test.go

It doesn't seem to exist - I don't think it was ever committed to this
public repo.

Signed-off-by: Nic Cope <nicc@rk0n.org>

lib/proto/proto.go

@@ -72,9 +72,6 @@
 // are frozen.
 //
 // TODO(adonovan): document descriptors, enums, message instantiation.
-//
-// See proto_test.go for an example of how to use the 'proto'
-// module in an application that embeds Starlark.
 package proto
 
 // TODO(adonovan): Go and Starlark API improvements: