config: move config_from_gitmodules to submodule-config.c

config: move config_from_gitmodules to submodule-config.c

The .gitmodules file is not meant as a place to store arbitrary
configuration to distribute with the repository.

Move config_from_gitmodules() out of config.c and into
submodule-config.c to make it even clearer that it is not a mechanism to
retrieve arbitrary configuration from the .gitmodules file.

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Acked-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>