odb: drop gaps in object info flag values

odb: drop gaps in object info flag values

The object info flag values have a two gaps in their definitions, where
some bits are skipped over. These gaps don't really hurt, but it makes
one wonder whether anything is going on and whether a subset of flags
might be defined somewhere else.

That's not the case though. Instead, this is a case of flags that have
been dropped in the past:

  - The value 4 was used by `OBJECT_INFO_SKIP_CACHED`, removed in
    9c8a294a1a (sha1-file: remove OBJECT_INFO_SKIP_CACHED, 2020-01-02).

  - The value 8 was used by `OBJECT_INFO_ALLOW_UNKNOWN_TYPE`, removed in
    ae24b032a0 (object-file: drop OBJECT_INFO_ALLOW_UNKNOWN_TYPE flag,
    2025-05-16).

Close those gaps to avoid any more confusion.

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