Remove RetriableStream flush workaround

In https://github.com/grpc/grpc-java/pull/9360 I added this workaround:

https://github.com/grpc/grpc-java/blob/7c73baa450c87e6a6f767d7451c46ca256e5158a/core/src/main/java/io/grpc/internal/RetriableStream.java#L565-L568

A flush() must always follow a write (but other flushes and calls could be in-between). I tried to flush everywhere but it didn't work at the time. See also #9340. It is possible there were interactions with https://github.com/grpc/grpc-java/issues/9547 which prevented it from working and just trying again may work.