raygun: Remove redundant stream macros in RayGun::attach

[dariusc93]

In warp-ipfs, RayGun::attach (or specifically, MessageStore::attach) makes use of stream macros from async-stream, however there are parts that are performing a single yield that are pushed into the SelectAll which were leftover from when these streams had additional logic. While this works fine as is, these single yield macros can be seen as redundant (which they are) and could effectively be replaced with futures::stream::once.