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>