Replace some uses of eval

Replace some uses of eval

There are a number of places where eval is used unnecessarily to get
or set the value of a symbol.
* lisp/calendar/calendar.el (diary-date-forms): Use default-value in
custom setter.
* lisp/desktop.el (desktop-clear): Use set-default instead.
* lisp/international/ogonek.el (ogonek-read-encoding): Use
symbol-value.