busybox

vi: basic undo feature shouldn't depend on yankmark

Currently basic undo functionality with the 'u' key depends on
FEATURE_VI_YANKMARK. These two features are separate, so we can remove this
dependency.

Signed-off-by: Andrew Fuller <abf@google.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>