Fix unused parameter warnings in arena_free
Fix unused parameter warnings in arena_free

The size parameter is only used in cases where the compiler supports
sized delete, so when that's not available we need to specifically cast
it to void to prevent a warning.

google/protobuf,Livefyre/protobuf,google/protobuf,Livefyre/protobuf,google/protobuf,google/protobuf,Livefyre/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,Livefyre/protobuf,google/protobuf,Livefyre/protobuf,google/protobuf,google/protobuf