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.
Fixesgolang/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>