greybus: connection: remove broken protocol-version handling

Remove the broken legacy protocol-version handling from core and move it
to the legacy driver instead.

Note that version handling has always been broken as legacy protocols
were looked up and bound to connections before the connections were
enabled and version negotiation could take place (over that very
connection). Document that in the legacy driver as well.

Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>