message: attribute_context: do not include unused base.proto.
  Fix a build warning:
  INFO: From ProtoGenValidateCcGenerate external/envoy_api/envoy/service/auth/v2/attribute_context.pb.h:
  envoy/service/auth/v2/attribute_context.proto: warning: Import envoy/api/v2/core/base.proto but not used.
diff: @@ -6,7 +6,6 @@ option java_outer_classname = "AttributeContextProto";
  option java_multiple_files = true;
  option java_package = "io.envoyproxy.envoy.service.auth.v2";
  -import "envoy/api/v2/core/base.proto";
  import "envoy/api/v2/core/address.proto";
  import "google/protobuf/timestamp.proto";
  