Remove gogo from .proto files

While gogo isn't actually used, it is still referenced from .proto files
and its corresponding Go package is imported from the auto-generated
files.

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

api/Protobuild.toml

@@ -8,11 +8,6 @@ generators = ["go", "go-grpc"]
   # treat the root of the project as an include, but this may not be necessary.
   before = []
 
-  # Paths that should be treated as include roots in relation to the vendor
-  # directory. These will be calculated with the vendor directory nearest the
-  # target package.
-  packages = ["github.com/gogo/protobuf"]
-
   # Paths that will be added untouched to the end of the includes. We use
   # `/usr/local/include` to pickup the common install location of protobuf.
   # This is the default.
@@ -46,5 +41,4 @@ prefix = "github.com/containerd/containerd/api"
 target = "next.pb.txt"
 ignore_files = [
 	"google/protobuf/descriptor.proto",
-	"gogoproto/gogo.proto"
 ]