odb: drop forward declaration of `read_info_alternates()`

odb: drop forward declaration of `read_info_alternates()`

Now that we have removed the mutual recursion in the preceding commit
it is not necessary anymore to have a forward declaration of the
`read_info_alternates()` function. Move the function and its
dependencies further up so that we can remove it.

Note that this commit also removes the function documentation of
`read_info_alternates()`. It's unclear what it's documenting, but it for
sure isn't documenting the modern behaviour of the function anymore.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>