D-Bus: Remove the obsolete notes from wpas_dbus_error_unknown_error()

D-Bus: Remove the obsolete notes from wpas_dbus_error_unknown_error()

Commit 6aeeb6fa21bc072ba92ce9423ba5c0417e8c0bf5 ('dbus: clean up new
D-Bus interface getters and setters') redesigned the property
getter/setter calls in a way that made the
wpas_dbus_error_unknown_error() note about message being NULL in some
cases obsolete. All the remaining callers are from method handler
functions that must have a valid message. Remove the obsolete notes and
unnecessary messsage == NULL check.

Signed-off-by: Jouni Malinen <j@w1.fi>