Remove redundant slot validation in eieio-persistent-read

Remove redundant slot validation in eieio-persistent-read

Actual object creation (in `make-instance') will later run all slot
values through cl-typep, which does a better job of validation. This
validation is redundant, and slows the read process down.

* lisp/emacs-lisp/eieio-base.el (eieio-persistent-fix-value): Rename
from `eieio-persistent-validate/fix-slot-value', as we no longer
validate, and we don't care about the slot definition.
(eieio-persistent-slot-type-is-class-p): Delete function.
(eieio-persistent-convert-list-to-object): Still call
`eieio--full-class-object', to trigger an autoload if necessary, but
discard the return value.