Don't consider obsolete commands for completion in some cases

Don't consider obsolete commands for completion in some cases

* lisp/simple.el (read-extended-command): Exclude obsolete commands
that are either lacking a 'current-name' or were obsoleted in a
previous major version (bug#43300).

(There's been some back and forth here.  Obsolete commands used to be
treated normally for completion, and then they were removed.  Then
they were put back again, but annotated with what they were
obsoleting.  There was some pushback on this change, so this latest
changes is a compromise between the last two states.)