multisession.el: Avoid old specializer syntax

multisession.el: Avoid old specializer syntax

Avoid the use of the old (eql VAL) specializer, preferring the new
(eql EXP) form.

* lisp/emacs-lisp/multisession.el (multisession-backend-value)
multisession--backend-set-value, multisession--backend-values)
multisession--backend-delete, multisession-backend-value)
multisession--backend-set-value, multisession--backend-values)
multisession--backend-delete): Quote the symbol passed to `eql` specializer.