Use upstream implementation of insecure credentials
This lets us get rid of code that we copied out of a "future" version of grpc-go. We are now up-to-date enough that grpc-go has the insecure credentials.
Part of gitlab-com/gl-infra/scalability#1124 (closed). It's not strictly necessary to clean this up now but it makes working in the backchannel
package easier.
Edited by Jacob Vosmaer