submodule-config: allow submodule_free to handle arbitrary repositories

submodule-config: allow submodule_free to handle arbitrary repositories

At some point we may want to rename the function so that it describes what
it actually does as 'submodule_free' doesn't quite describe that this
clears a repository's submodule cache.  But that's beyond the scope of
this series.

While at it remove the extern key word from its declaration.

Signed-off-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>