Some ssh-agent clients expect the server to support remove all messages for protocols 1 & 2 and error if protocol 1 support is missing. This adds a null-op implementation of the remove all message in similar fashion to the existing list all message support. Fixes golang/go#15159 Change-Id: I9389885d89c9147f3e10850893bba6ed7d0a4e82 Reviewed-on: https://go-review.googlesource.com/21468 Reviewed-by: Han-Wen Nienhuys <hanwen@google.com> Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>