elisp-mode.el: Use `handler-bind` instead of `debug-on-error`

elisp-mode.el: Use `handler-bind` instead of `debug-on-error`

* lisp/progmodes/elisp-mode.el (elisp-enable-lexical-binding):
Don't get fooled by a global binding of `lexical-binding` to t.
(elisp--eval-last-sexp-fake-value): Delete var.
(elisp--eval-defun): Don't let-bind `debug-on-error` since it's
already arranged by the only caller.
(eval-last-sexp, eval-defun): Use `handler-bind` instead of
`debug-on-error`.