submodule: remove submodule_config callback routine

submodule: remove submodule_config callback routine

Remove the last remaining caller of 'submodule_config()' as well as the
function itself.

With 'submodule_config()' being removed the submodule-config API can be
a little simpler as callers don't need to worry about whether or not
they need to overlay the repository's config on top of the
submodule-config.  This also makes it more difficult to accidentally
add non-submodule specific configuration to the .gitmodules file.

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