sequencer: switch non-recursive merges over to ort

sequencer: switch non-recursive merges over to ort

The do_recursive_merge() function, which is somewhat misleadingly named
since its purpose in life is to do a *non*-recursive merge, had code to
allow either using the recursive or ort backends.  The default has been
ort for a very long time, let's just remove the code path for allowing
the recursive backend to be selected.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>