This has been broken since PostgreSQL 12 and was probably never really
used.  PostgreSQL 12 added an analogous HEAPAMSLOTDEBUGALL, which
still works right now, but it's also not very useful, so remove that
as well.

Discussion: https://www.postgresql.org/message-id/flat/645c0646-4218-d4c3-409a-a7003a0c108d%402ndquadrant.com