Revert "Teach DSM registry to ERROR if attaching to an uninitialized entry."

Revert "Teach DSM registry to ERROR if attaching to an uninitialized entry."

This reverts commit 1165a933aa (and the corresponding commits on
the back-branches).  In a follow-up commit, we'll teach the
registry to retry entry initialization instead of leaving it in a
permanently failed state.

Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/E1vJHUk-006I7r-37%40gemulon.postgresql.org
Backpatch-through: 17