submodule: remove submodule.fetchjobs from submodule-config parsing

submodule: remove submodule.fetchjobs from submodule-config parsing

The '.gitmodules' file should only contain information pertinent to
configuring individual submodules (name to path mapping, URL where to
obtain the submodule, etc.) while other configuration like the number of
jobs to use when fetching submodules should be a part of the
repository's config.

Remove the 'submodule.fetchjobs' configuration option from the general
submodule-config parsing and instead rely on using the
'config_from_gitmodules()' in order to maintain backwards compatibility
with this config being placed in the '.gitmodules' file.

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