Make EIEIO ':accessor' behave like ':reader' when reading (bug#66938)

Make EIEIO ':accessor' behave like ':reader' when reading (bug#66938)

Clones of instances of subclasses of 'eieio-instance-inheritor' didn't
delegate to their ':parent-instance' field when reading object fields
using ':accessor'.

* lisp/emacs-lisp/eieio.el (defclass): Remove 'slot-boundp' check for
:accessor's getter
* test/lisp/emacs-lisp/eieio-tests/eieio-tests.el
(eieio-test-use-accessor-function-with-cloned-object): New test.

Copyright-paperwork-exempt: yes