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>