Remove references to netty so that using grpc with shaded netty is possible

[garrettjonesgoogle]

[garrettjonesgoogle]

There are no more references in the main code, but com.google.api.gax.testing has some references in MockServiceHelper and LocalChannelProvider.

[garrettjonesgoogle]

Since we aren't shading Netty until after GA, this isn't blocking GCJ GA.

[garrettjonesgoogle]

This was done in #469 .