Remove sha1.

Remove sha1.

Server and client prefer sha256 in the authentication protocol since
paraslash-0.7.0, while 0.6.x clients and servers only know about sha1. This
patch drops the compatibility code which enabled old clients to connect to
new servers and vice versa.

Now the client code no longer checks whether the sha256 feature is supported,
but requests and uses the feature unconditionally. The server still announces
the sha256 feature and accepts the corresponding feature request, but uses
sha256 unconditionally.

With this approach older 0.7.x clients are still compatible to current
server versions and current clients can still talk to servers running older
0.7.x versions.