encoding/protojson: simplify Duration formatting
encoding/protojson: simplify Duration formatting

Simplify the implementation by reducing the number of branches.

Change-Id: I6e2ffee0fc2d77f7e2a70f76e03d081f4fc0e99d
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/247459
Reviewed-by: Herbie Ong <86f376bf42b6daa37c768c730acd318ab6855de4@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go