merge-tree: remove unused df_conflict arguments

merge-tree: remove unused df_conflict arguments

merge_trees_recursive() stores a pointer to its parameter df_conflict in
its struct traverse_info, but it is never actually used.  Stop doing
that, remove the parameter and inline the function into merge_trees(),
as the latter is now only passing on its parameters.

Remove the parameter df_conflict from unresolved_directory() as well,
now that there is no way to pass it to merge_trees_recursive() through
that function anymore.

Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>