Do not use weak import

According to https://github.com/protocolbuffers/protobuf/issues/9184

> Weak fields are an old and deprecated internal-only feature that we never
> open sourced.

This blocks us to upgrade protoc.

Fixes #6232.

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

runtime/v2/runc/options/next.pb.txt

@@ -161,6 +161,5 @@ file {
   options {
     go_package: "github.com/containerd/containerd/runtime/v2/runc/options;options"
   }
-  weak_dependency: 0
   syntax: "proto3"
 }