remove remaining reference to evil-symex-state

This is now moved into Rigpa, so it doesn't make sense to check it
within symex. We can also now just rely on the Lithium mode
`symex-editing-mode` (which is just an ordinary Emacs minor mode) to
check whether symex mode is active.

symex-misc.el

@@ -356,8 +356,7 @@ measure them anywhere, we do this traversal in O(n)."
 
 (defun symex-select-nearest-advice (&rest _)
   "Advice to select the nearest symex."
-  (when (and (fboundp 'evil-symex-state-p)
-             (evil-symex-state-p))
+  (when symex-editing-mode
     (symex-user-select-nearest)))
 
 (defun symex--selection-side-effects ()