unpack-trees: drop name_entry from traverse_by_cache_tree()

unpack-trees: drop name_entry from traverse_by_cache_tree()

We pull the names from the existing index rather than the tree entry,
which is after all the point of this function. Let's drop the unused
"names" parameter.

Note that we leave the "nr_names" parameter, as it tells us how many
trees we are traversing (and thus how many index stages to set up).

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>