attribute_context: do not include unused base.proto. (#6796)

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.

Signed-off-by: Ismo Puustinen <ismo.puustinen@intel.com>

Mirrored from https://github.com/envoyproxy/envoy @ 16e85b1ffc4fceb3c1c3857b96862937126468a8

envoy/service/auth/v2/attribute_context.proto

@@ -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";