unpack-trees: don't respect submodule.update

unpack-trees: don't respect submodule.update

The 'submodule.update' config was historically used and respected by the
'submodule update' command because update handled a variety of different
ways it updated a submodule.  As we begin teaching other commands about
submodules it makes more sense for the different settings of
'submodule.update' to be handled by the individual commands themselves
(checkout, rebase, merge, etc) so it shouldn't be respected by the
native checkout command.

Also remove the overlaying of the repository's config (via using
'submodule_config()') from the commands which use the unpack-trees
logic (checkout, read-tree, reset).

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>