Consider removing ExperimentalTag for bidirectional streams

[dbolduc]

The presence of the ExperimentalTag on read-write streams is leading to unnecessary confusion. (See #10285 / #10305).

We originally marked these APIs as experimental, because the interface is a bit tricky to use. Most of these bidirectional streams have service-specific behavior. (Googlers can see go/cloud-cxx:generated-bidir-rpcs).

We were hopeful that we could improve the API (maybe by offering hand-crafted APIs). We might not be able to. If we are able to, we can always do so by adding a new API (instead of replacing the old API). Having multiple ways to call the same thing may lead to some confusion, but I claim it is less confusion than the ExperimentalTags cause.

We should consider removing ExperimentalTag for bidirectional streams.