builtin/fsck: drop `fsck_head_link()`

builtin/fsck: drop `fsck_head_link()`

The function `fsck_head_link()` was historically used to perform a
couple of consistency checks for refs. (Almost) all of these checks have
now been moved into the refs subsystem. There's only a single check
remaining that verifies whether `refs_resolve_ref_unsafe()` returns a
`NULL` pointer. This may happen in a couple of cases:

  - When `refs_is_safe()` declares the ref to be unsafe. We already have
    checks for this as we verify refnames with `check_refname_format()`.

  - When the ref doesn't exist. A repository without "HEAD" is
    completely broken though, and we would notice this error ahead of
    time already.

  - In case the caller passes `RESOLVE_REF_READING` and the ref is a
    symref that doesn't resolve. We don't pass this flag though.

As such, this check doesn't cover anything anymore that isn't already
covered by `refs_fsck()`. Drop it, which also allows us to inline the
call to `refs_resolve_ref_unsafe()`.

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